The same changes, Changes continued to recent times. i.e., the retreat of the miocene sea from the tertiary continent and finally its separation into several lake-basins (the AraloCaspian), continued long after the deposition of the estuarine strata along the margin of the Turkistán highlands. With the change of outline of these lake-basins and the diminution of rainfall and consequent decrease in volume of the drainage flowing from the Afghan highlands, begin to appear aerial formations, which near the margin of the hills are seen to be intercalated between the fluviatile deposits of the upper tertiaries. In pliocene times already began the accumulations of vast deposits of loess, whose aerial origin is clearly shown in the scarps which every Turkistán river cuts through it in its northward flow. Aerial formations. The accumulation of loess is still going on over a large portion of Central Asia including Northern Afghanistán. The steppes owe their origin to the nature of the recent deposits of loess. Every one of the wider valleys of Eastern Khorassan and the Herat valley are partly filled with loess accumulations. The grass-covered downs (Chúll) of Badghis, Maimana, and Afghan Turkistán are nothing but widespread loess deposits. In South-western Afghanistán the conditions are similar. There the gradually drying-up inland basin of the Helmund is under the influence of air-currents which deposit vast thicknesses of loess on both banks of the Helmund and in Seistán; they rest on stratified deposits of clay, sands and conglomerate, which are lithologically identical with the Chull deposits of Turkistán. For the description of the Biluchistán ranges, the Búgti hills, and the Sulimán range I refer to the descriptions given in the publications of the Geological Survey of India. I found it impossible to separate the various intrusive igneous rocks on the small Igneous rocks. scale map which accompanies this paper. I believe the igneous rocks of Afghanistán and North-eastern Khorassan belong, broadly speaking, to four different epochs. They are : 1. The oldest igneous rock of Afghanistán I believe appears as interbedded trap and melaphyre in the lowest beds of the plant series, which I have correlated with the permian of the Araxes and the Himalayas and the Talchirs of India. They are well seen in the Herat sections and in the north-western extension of the Yaktán range of Khorassan. 2. Enormous outbursts of traps and porphyritic rocks appear as intrusive and partly interbedded masses in the red-grit group. They are particularly well seen C. L. Griesbach: Mem. Vol. XVIII. 1 W. T. Blanford: Mem. Vol. XX, pt. 2. W. T. Blanford; Mem. Vol. XX, pt. 2. 3 W. T. Blanford: Mem. Vol. XX, pt. 2. P. 175. C. L. Griesbach: Records, Vol. XVII, pt. 4, in the Herat province; the Ardewán, Marbich, Band-i- Bába, and Zurmust passes lead over sections through these late jurassic traps. The upper beds of the red-grit group are almost entirely made up of fragments of igneous rocks and great thicknesses of ash-beds swell out the total of this group enormously. 3. Structurally the most important of the igneous rocks of Afghanistán are the syenitic granite and trap outbursts of post-cretaceous times. They form dykes and bosses in Khorassan and the Herat province (Davendar) and form most probably a continuous belt through Central Afghanistán, I met them again in great force in the Hindu Kush, which, with its parallel chains north and south, is nearly entirely composed of these rocks. They are seen to penetrate the upper cretaceous limestone, which is then frequently converted into white marble near the contact. I believe the outburst to date either from late cretaceous or eocene times. To it belong the post-cretaceous syenites and traps of Kandahar and the desert south of the Helmund. 4. In Eastern Khorassan appear trachytes and rhyolites (north-west of Nishapúr) which have intruded between strata of nummulitic limestone and therefore can hardly be older than middle tertiary. Summary. The following facts may again be summarized. 1. The carboniferous group shows great points of resemblance, both lithologically and paleontologically, over the entire distance from the Caucasus to the North-west Himalayas. It is a purely marine formation, and pelagic conditions seem to have prevailed in the Caspian region, Northern Persia, Afghanistán, and the Himalayan area. 2. At the close of the carboniferous period began a shallowing of the sea over the greater part of Central Asia, including Northern Persia and Afghanistán, which more or less continued to neocomian times, when the sea altogether retreated from large tracts of Central Asia, including Afghan Turkistán. During that time littoral deposits were laid down along a coast line which seems to have agreed more or less with the present direction of the Central Asian watershed. In the adjoining areas, of Asia Minor and the Himalaya, pelagic conditions continued. 3. In upper cretaceous times a great overlap of the sea began and extended to lower miocene times. 4. After the deposition of the lower miocene the sea began to retreat gradually from the coast-lines, not only in the Central Asian area, but also in Sind and Biluchistán, and estuarine and freshwater deposits were being laid down conformably on the marine miocene beds. The retreat of the miocene seas was continued in late tertiary times and in fact the same changes are still proceeding at this moment. In place of the estuarine deposits huge accumulations of freshwater and aerial formations took place over the greater part of Central Asia, Persia, and our Sind frontier. Notes on the Microscopic structure of some specimens of the Rájmahál and Deccan traps, by COLONEL C. A. MCMAHON., F.G.S. (Received December 23, 1886.). Igneous rocks of the Rájmahal group: Augite-Andesites. This rock has been considerably decomposed by the percolation of water, and has been reduced to a condition that reminds me very much of the traps of Mandi and Darang, in the Himalayas. The matrix of the amygdaloid consists of a network of felspar prisms, starred about in a muddy-looking, and almost opaque, base of greenish-brown colour. In reflected light this is seen to be studded at regular intervals with fine fibres of magnetite which radiate in all directions. The slice is dappled here and there with an amorphous red product; remains of augite are also visible, but this mineral has been altered almost out of recognition. All the felspar belongs to the triclinic system, and none of it appears in the form of porphyritic crystals 1-441.-Amygdaloid. Dubrajpur. The matrix of this specimen is composed of triclinic felspar, augite, and magnetite. The felspar occurs both in porphyritic crystals, and in small but regular prisms. If the former were left out of consideration, the smaller felspars and the augite would be in about equal proportions to each other. The augite is in a granular condition; that is to say, it does not present crystallographic outlines. A zeolite, varying from white to yellowish-red in colour, stops the amygdules, and invades the substance of the rock, forming very numerous lacunæ, with strongly marked marginal borders. The inner portions of many of the large felspars have also been replaced by this product of decomposition. With this exception the rock presents a fairly fresh appearance. Among the felspar crystals, crosses formed by baveno twins are common; and whilst twinning on the albite plan is visible in nearly all the felspars, pericline macles are also to be seen here and there. Augites occur in both these slices in somewhat large crystals as well as in smaller granules, and a few of them present rather good external shapes. Twinning is to be observed in some of them, but it is not at all common. The augite and felspar appear, on the whole, to have crystallized simultaneously, but instances are not wanting to show that some individuals of both minerals crystallized in advance of others. Thus these slices present examples of twinned and wellformed augites, and groups of augites, imbedded in rather large felspars; and also of augites formed upon felspar prisms. In other cases the growth of the latter mineral has apparently been stunted by the formation of crystals of pyroxene by their side. 1 The numbers given are those of the Geological Survey of India. This feature, viz., the simultaneous formation of augite and felspar, and the enclosure of augite by felspar, and of felspar by augite, was noted as a characteristic of the Bombay basalts in my paper on those lavas, see Records XVI, pp. 43, 46, and plates. I may further draw attention to the fact, in passing, that the inclusion of groups of well-formed augites in large felspars, noted above, shows conclusively that we must not jump to the conclusion that all large felspars porphyritically imbedded in a finer grained ground-mass necessarily belong to a different "generation," and had their birth considerably in advance of the smaller crystals of the ground-mass. The porphyritic and zoned felspars were doubtless formed before the small prisms of felspar; but the interval between them need not have been great in point of time, and the birth of both may have been the result of cooling when the lava began to consolidate after it had flowed from the mouth of the volcanic crater. At any rate, it is clear that some small crystals of augite were formed in the magma prior to the crystallization of the large felspars, and we may reasonably infer that the crystallization of the whole mass had begun when the porphyritic felspars were formed. This inference is also supported by a further fact observed in slice No. 436; namely, that there is a gradation from the felspars of largest to those of smallest size, and not an abrupt transition from large porphyritic felspars to small lath-shaped prisms. In No. 436, magnetite is very abundant. The products of decomposition are, in No. 436, a greenish-brown; and in No. 440, a substance that is of greenish-brown colour in some places, and yellowish-red in others. This cannot be traced directly, or indirectly, to the alteration of augite; on the contrary it fills the rôle of a glassy base, and possibly represents the uncrystallized residuum. The pyroxene is very fresh, but occasionally inner portions of the large felspars are replaced by the secondary product of decomposition, above alluded to, which is not green enough to be called viridite. These probably represent portions of the base caught up in the felspars. Occasionally in No. 436 the greenish-brown matter appears in rounded forms, suggestive at first sight of pseudomorphs after olivine, but this appearance is, I think, delusive; and the hypothesis that might be based on it is refuted by the general behaviour of this greenishbrown matter and by other considerations. This greenish-brown material apparently forms the base in which the constituent minerals of the rock are imbedded. Moreover these slices contain no serpentine, or serpentinous viridite, and no trace of Maschen Structur due to the deposition of opacite, or magnetite, round the edges or along the cracks in olivine crystals, which often betray the original presence of that mineral after its substance has been converted into serpentine. The conclusion at which I have arrived, after a careful study of the Rájmahál slices is that there is not a trace of olivine in any of them. 1-436 Rájmahál Hills. 1-440. "Ball trap." Motijharna. Viewed macroscopically these are very basaltic-looking specimens, and they would no doubt ordinarily be called basalts. Under the microscope they are seen to belong to exactly the same class of rock as No. 441, described above. Indeed, the difference between them is not one of original structure, or composition; but consists entirely. in the relative progress of decay. The amygdaloid was more exposed to the infiltration of water than the less porous lavas, and has necessarily suffered more. As a consequence the zeolites so abundant in the amygdaloids are entirely absent from these two specimens. As in No. 441, the two rocks under description consist of a mixture of triclinic felspar, augite, and magnetite; and as in the Dubrajpur lava, porphyritic crystals of felspar are imbedded in a ground-mass composed of small prisms of that mineral, and granular augites. In No. 440 the crystals of the ground-mass are relatively smaller, as compared with the large felspars, than in No. 436. Zonal structure, characteristic of the andesitic type of rocks,' is common in the porphyritic felspars. I-446. Betia Hill. Enstatite-hornblende-diorite.. This is a very interesting specimen. It is composed of the following minerals, namely, triclinic felspar, enstatite, hornblende, magnetite, quartz, apatite, and a little mica. In structure it is perfectly granitic and holocrystalline. The felspar, judging from its optical characters, is labradorite. It is considerably "schillerized” (Judd), and the included matter is arranged in such regular lines, that in some cases, when low powers are used, it imparts the appearance of fine striation to the crystals. Occasionally the felspar is so crammed with colourless globulites, and microliths, as to impart a graphic appearance to it similar to some of the felspar in the Dalhousie and Sutlej Valley gneissose granites (query, is this eucryptite, or an allied mineral -see Dana's 3rd Appendix, p. 113); at other times rounded microliths of hornblende occur. The enstatite is about as abundant as the hornblende; it is intimately associated with it, and sometimes it is surrounded by a fringe of the latter mineral. Magnetite is abundant, and is usually imbedded in the hornblende. There is not much quartz; and it is evidently the residuum left after the crystallization of the other minerals. It contains some liquid cavities with bubbles. Apatite is abundant, and is present both in the form of slender microliths and also in good-sized prisms. Mica cannot be detected in the thin slice, but a small packet of undoubted mica was observed on a macroscopic examination of the hand specimen with a pocket lens; it is of dark reddish-brown colour, and an examination of a flake in convergent polarized light showed that it is one of the micas that used to be referred to the hexagonal system. Andesite. No. 14-19. Sendurgusi Hill. Sp. G. 265. A compact lava of pinkish colour. It is very rough on the fractured surface, the roughness being produced by its fine-grained porosity; indeed so porous is the rock that the hand specimen, on being plunged into water, continued to give forth streams of air-bubbles for a long time. Owing to the air contained in the specimen, some of which was probably unable to escape, the specific gravity given above is, I dare say, a little too high; but the point is not material as andesites range as low as 2'54o and this specimen cannot be much, if at all, lower than 2'6. 'Judd : Quar. Jour. Geol. Soc. xlii, 426. Teall: Geol. Mag., 1883, p. 107. |