25. On the ORIGIN of SLICKENSIDES, with REMARKS on SPECIMENS from the CAMBRIAN, SILURIAN, CARBONIFEROUS, and TRIASSIC FORMATIONS. By D. MACKINTOSH, Esq., F.G.S. (Read February 24, 1875.)

[Abstract]

WHILE admitting that the sides of many fractures have been coated with foreign substances, the author's observations led him to believe that true slickensides, or polished and smoothly striated surfaces, have been produced by the movement of one face of rock against another, accompanied by partial fusion. In the case of many of the specimens, the surface had apparently been metamorphosed by heat resulting from pressure, as the whitened and hardened coating graduated imperceptibly into the ordinary structure of the rock. In some cases sandstone had apparently been converted into quartzite. He ventured to bring "the theory of metamorphism by heat in the case of many slickensided surfaces" before the Society, in expectation that it might lead to further investigation.

DISCUSSION.

Mr. SORBY said that while some facts seem to indicate the action of heat, there are many reasons which apparently show that the operation of that agent is unlikely. He believed that many actions and causes which we do not at present understand have been at work in producing slickensides. The subject deserved careful study.

Mr. FORBES could not agree with the author of this paper that the phenomena of slickensides were due to a partial fusion of the sides, since he had observed some of the very finest examples of slickensides on pyrites in the great deposits of Rio Tinto; and as this mineral was in a perfectly unaltered condition, it is evident that no heat reaching to any thing like its fusing-point could have been developed; nor could this be the case in limestones or in hydrated rocks, both of which would be altered in chemical composition if strongly heated. It would also be the same with the slickensides seen in the cinnabar from Borneo and Almaden, which mineral sublimes if exposed to even a comparatively gentle heat.

Prof. TENNANT said that at Matlock galena showed the same

appearances.

* Since this paper was read and discussed, the author has seen reason for restricting the term "partial fusion" to certain cases. He has lately found that Mr. John Aitken, F.G.S., of Bacup, advocated metamorphism amounting to vitrification by heat, as the cause of slickensides in 1873 (Proc. Manchester Geol. Soc. vol. xii.), and that Professor Marsh, of Yale College, U. S., in 1867, regarded slicken sides as "friction-marks," and the accompanying coating as "merely a portion of the rock finely divided and compressed" (Proc. Am. Association of Science).-D. M., June 1875.

Mr. DowKER remarked on the production of slickensides in clay from movements due to alternate expansion and contraction by moisture and drought. He also showed the occurrence of a similar phenomenon in a crystalline mass found in a cavity of the Chalk. Mr. KоCH remarked that slickensides may be seen in coal, in which case heat could have had no action.

Prof. RUPERT JONES inquired why mere rubbing might not have been the cause as in artificial polishing.

Prof. SEELEY referred to some examples of artificial slickensides caused by striking and rubbing.

26. NOTES on the COMPARATIVE MICROSCOPIC ROCK-STRUCTURE of some ANCIENT and MODERN VOLCANIC ROCKS. By J. CLIFTON WARD, Esq., Assoc. R.S.M., F.G.S., of the Geological Survey of England and Wales. (Read November 4, 1874.)

II. Examples of Microscopic Rock-structure among Modern Lavas.

1. Trachyte, Solfatara, near Naples (figs. 1 & 2).

2. Leucitic Basalt, Torre dell' Annunziata, Naples (fig. 3).

3. Leucitic Basalt, near Torre del Greco, Naples (figs. 4 & 5).

4. Leucitic Basalt, Albano, near Rome.

Summary.

III. Examples of Microscopic Rock-structure among the Lavas and Ashes of Wales.

a. Aran and Arenig Felstones.

1. Aran Mowddwy (figs. 16 & 17).

2. Aran Benlynn (fig. 18).

3. Y Foel-ddu.

4. Efridd Trawscoed.

b. Snowdon Felstones.

5. Llanberis Route, Snowdon (fig. 19).

6. Glaslyn, Snowdon.

7. Llyn Cwlyd, Capel Curig.

8. Llanberis Route, Snowdon (Slaggy Felstone, fig. 20).

c. Ashes of Snowdon, Arenig, and the Arans.

1. Glaslyn, Snowdon (fig. 21).

2. The Arans (fig. 22).

3. The Arenigs.

Summary.

IV. Examples of Microscopic Rock-structure among the Lavas and Ashes of

Cumberland.

Introduction.

a. Cumberland Lavas.

1. Eycott Hill (fig. 6).

2. Brown Knotts, near Keswick (fig. 7).

3. Latterbarrow, Wastwater (fig. 8).

General Remarks.

b. Cumberland Ashes and Felstone-like Rocks.

1. Typical Ash, Steel Fell, Wythburn (fig. 10).

2. Altered Slate, Craig Wen, Capel Curig.

3. Bleaberry Fell, near Keswick (fig. 9).

4. Hart Side, North of Glencoindale Head (fig. 13).

5. Base Brown, Borrowdale (figs. 11 & 12).

6. Great Gable (fig. 14).

7. Rigghause End, Vale of St. John (figs. 15 & 16).

c. Manner of Metamorphism.

d. Period of Metamorphism.

e. Parallel Metamorphism of Welsh Lavas and Ashes.
f. Classification of Cumberland Lavas.

g. Conclusion.

Introduction.-The object of the present paper (submitted to the Society with the permission of the Director-General of the Geological Survey) is, by comparing the microscopic rock-structure of several distinct groups of volcanic rocks, to gain an insight into the original structure of some of the most ancient, and therefore probably the most altered, members of the volcanic series. My desire has been mainly to note the relations of the component minerals to one another, and thus glean facts in the history of the rocks which they compose. For this purpose I have endeavoured by coloured drawings to render as truthfully as possible the microscopic structure of most of the examples brought forward. I have to acknowledge the very kind and willing assistance rendered by Mr. Allport in helping me to take up and prosecute to a small degree this line of research.

I. HISTORY OF THE SUBJECT*.

Very little, comparatively, has as yet been done in England on this subject; but perhaps some of the most valuable work that has as yet been effected in microscopic geology is the result of the labours of our countrymen Sorby and Allport.

In 1858, Sorby brought out his celebrated paper on the "Microscopical Structure of Crystals" +. He describes the glass, stone, and gas- or vapour-cavities in the minerals of the pitchstones of Arran, the lavas of Vesuvius, and some of the basaltic rocks of Scotland. Remarking on the great alteration which has been effected in the ancient trap-rocks during the course of ages, he nevertheless observes (p. 479), "The characteristic structure of the minerals of which ancient trappean rocks are composed is therefore so analogous to, or even identical with, that of the constituents of modern lavas, that the purely igneous origin of these ancient lavas appears to me to be completely established; but, at the same time, their present aspect is often to a very great extent due to the subsequent action of water." With regard to rocks formed at a high temperature, the author's chief conclusions are as follows:-"At one end of the chain are erupted lavas, indicating as perfect and complete fusion as the slags of furnaces; and at the other end are simple quartz veins, having a structure precisely analogous to that of crystals deposited from water. Between these there is every connecting link; and the central link is granite" (p. 496).

In 1867 an article appeared upon "the Microscope in Geology"‡, written by Mr. David Forbes. The author shows how the different minerals in volcanic rocks may be distinguished from each other by means of the microscope, and gives figures illustrating the general microscopic structure of the following rocks :-lavas of Etna, Vesuvius, and Tahiti; pitchstone dyke of Arran; Staffordshire basalt, and "white horse" dykes; diorite from Chili; uralite porphyry from

* Reference will here only be made to those papers, or parts of papers, which bear upon volcanic rocks.

+ Quart. Journ. Geol. Soc. vol. xiv. pp. 453-500.

Popular Science Review, vol. vi. pp. 355-368.

Tyrol; quartz trachyte, and another volcanic rock from Peru; and some few metamorphic and sedimentary rocks.

The first of a series of valuable papers by Mr. Allport appeared in 1869 *; in it the author described the basalt of S. Staffordshire, and noted the presence of pseudomorphs after olivine. In the following year the same author contributed a paper on the "Basaltic Rocks of the Midland Coal-fields" +, showing that the traps of "Kinlet and Shatterford (west of Kidderminster), the Clee Hills, Little Wenlock (near the Wrekin, in Shropshire), Coalville (near Bardon Hill, in Leicestershire), and Matlock (in Derbyshire)," were all basaltic. In another paper, in the same year, upon the "Microscopical Examination of Rocks and Minerals"‡, Mr. Allport showed that the traps of the Warwickshire Coal-field contain hornblende instead of augite, and are therefore true diorites; at the end of the paper the author states his belief that the following results of microscopical examination will stand the test of further study :

"1. The mineral constituents of the melaphyres and other finegrained igneous rocks may be determined with certainty, a result which has not been attained by any other method of examination.

"2. The mineral constituents of the true volcanic rocks, and those of the old melaphyres, are generally the same.

"3. The old rocks have almost invariably undergone a considerable amount of alteration; and this change alone constitutes the difference now existing between them and the more recent volcanic basalts."

In 1871 Mr. Allport again contributed two papers, one "On the Microscopic structure and Composition of a Phonolite from the 'Wolf Rock""§, and the other on the " Relative Ages of Igneous Rocks"!. In the former, the structure of the rock is shown to be similar to the Tertiary Phonolites, and an analysis is given. In the latter the author concludes "that there is no essential difference between eruptive rocks of different geological epochs," and states, "I arrived at this conclusion more than three years since."

In the same year (1871) Prof. Zirkel published a valuable memoir entitled "Geologische Skizzen von der Westküste Schottlands"¶, in which he gave descriptions of the microscopic structure of many of the volcanic rocks of the western islands.

In 1872 follow papers by Mr. Allport" On the Microscopic Structure of the Pitchstones and Felsites of Arran"**. At p. 537 the bases of the pitchstones and the felsites are contrasted-the former being "a homogeneous glass without a trace of double refraction," the latter having a felsitic structure which " invariably exhibits double refraction," and "as either of the prisms is rotated, the mass appears to break up into variously coloured little patches, which gradually assume a more definite form as the axes approach to a right angle." None of the rocks described, however, seem to be con

*Geol. Mag. vol. vi. p. 115. Ibid. vol. vii. pp. 431-435. Ibid. vol. viii. pp. 448-450.

+ Ibid. vol. vii. pp. 159-162.

§ Ibid. vol. viii. pp. 247-250.

Zeitschr. d. deutschen geologischen Gesellschaft,' Jabrg. 1871, pp. 1–124. ** Geolog. Mag. vol. ix. pp. 1-9 and 536-545.