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it is found in places scattered over more than a square mile. It

has a Stonesfield-slate character.

This line of fault continued further would connect with the Atherstone and Nuneaton fault, and agrees with it in having its downthrow on the N.E. side.

As regards the amount of the throw it is not easy to speak confidently. At Hillmorton, however, it cannot be less than 120 feet; and at Brownsover it may be as much as 500 feet, if I am right in my view of the origin of the Oolitic cap to the Brownsover plateau.

There will be some points of special interest in determining the date of this fault, and its effects on the valley-system in its neighbourhood. It appears to me to have caused the existence of that extinct lake that I spoke of before, and to have altered the watershed. But it has left very few traces on the surface, which is here, for the most part, deeply overlaid with drift.

I may perhaps be permitted to mention that the Natural-History Society of the School has just constructed a model, on the scale of 6 inches to the mile, of our neighbourhood, and that any one who should feel inclined to come down and examine into the question, and thinks that an inspection of the model would assist him, will be made most welcome to all the assistance in our power.

DISCUSSION.

Prof. RAMSAY said that he was puzzled with the description of the sands and clays brought together vertically by a fault, and that he had never seen a fault of Postglacial age. He did not think the

phenomena described were due to a fault. With regard to the supposed Oolitic outlier, which was said to be of Stonesfield Slate resting upon Lower Lias, he did not see how such an arrangement could be brought about by a fault; for underneath the Stonesfield Slate in that district we naturally expect to find the Inferior Oolite series and the Upper Lias and Marlstone. The Oolitic mass had probably been conveyed to its present position by glacial action, as in many cases in the same parts of the country large masses of rocks occur in remarkable positions, as, for example, an erratic mass of Marlstone overlying Oxford Clay.

Mr. TATE stated, in confirmation of the remarks of Prof. Ramsay, with which he fully agreed, that in Yorkshire he had seen a great mass of Cleveland ironstone resting upon Upper Lias.

By D. C. DAVIES,

23. The РHOSPHORITE DEPOSITS of NORTH WALES.
Esq., F.G.S. (Read February 10, 1875.)

In the year 1863 a working miner picked up a piece of blacklooking stuff in a small ravine above Cwmgwynen farm-house, five miles west of the town of Llanfyllin, in North Wales. His discovery excited his curiosity, and led him to make some trial-holes in the ravine, in the hope that he might find the substance in bulk. He was successful in this; but the newly found substance perplexed both himself and his mining friends. It was strange to them all, except that some of them pronounced it manganese, and others rotten sulphur. At last, through the late Mr. Hope Jones, of Hooton, Cheshire, the mineral found its way into the hands of Dr. Voelcker, for analysis. Dr. Voelcker found that the best samples submitted to him yielded over 60 per cent. of phosphate of lime, and the most impure over 40 per cent.

At the meeting of the British Association, in Birmingham, in 1864, Dr. Voelcker called attention to the discovery, and rightly estimated the quantity of phosphate, in the property to which his attention had been directed, at over two million tons. Meanwhile preparations were being made for mining the newly discovered substance, and bringing it into the market. Searches, too, were made for the mineral on adjoining properties, which resulted in its discovery at Penygarnedd, to the N.E., and Pwllywrach, to the S.W. of Cwmgwynen, under similar conditions. The district was one well known to me previously; and I had occasion to visit the new workings repeatedly. In June 1867 I published in the Geological Magazine' a short account of the deposit, in which I gave illustrations of the manner in which it occurred, together with its relation to the surrounding strata. Early in the year 1872 I was asked to examine a similar deposit, which had just been discovered in a like manner at Craig Rhiwarth, in the Berwyn Mountains, between Llangynog and Bala, some six or seven miles to the west of Cwmgwynen. I at once, as I had anticipated, recognized in the new discovery the old bed, which, though now widely separated from the one first discovered, had evidently once been continuous with it.

Since then it has fallen to my lot to open up "a mine" at the latter place; and, partly as a consequence of my connexion with this mine, I have had occasion to visit several other places in North Wales where the deposit has been found. These are as far apart as the vicinity of the town of Llanfyllin, and the hills ranging west and north of Dinas Mowddwy.

I propose, in this communication, to combine with the remarks made in my paper of 1867 the result of the observations made by me during my closer acquaintance with the deposit during the last few years.

The deposit occurs in a bed whose geological position is at the top of the Bala Limestone and immediately under the shale by which

Fig. 1.-Section of Strata at Cwmgwynen Phosphate Mine, south west of Llangynog, North Wales.

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Bala Limestone (upper part).

that limestone is overlain (see section, fig. 1). The bed varies in thickness from 10 to 15 inches. It is black in colour from the graphite it contains; and its appearance is that of a number of concretions, which range in size from that of an egg to that of a fullsized cocoa-nut, closely packed and even running into each other, and cemented together by a black matrix. The concretions have often a polished appearance, which is also due to the presence of graphite; and frequently along the course of the bed the phosphorite is charged with concretions and crystals of sulphide of iron. Near the outcrop of the bed this becomes oxidized, and the deposit changes its black for a rusty appearance. The concretions, as will be seen by the analyses which accompany this paper, contain the most phosphate of lime, usually 64 per cent.; but the matrix also contains a portion; so that the average yield of the bed is about 46 per cent. Was it from the disintegration of a somewhat similar bed that the rolled and more widely diffused phosphatic nodules of Bedford, Cambridge, and Suffolk were derived?

The bed is underlain by a thin bed of crystalline limestone, which does not usually exceed 6 inches in thickness, though there are a few exceptions to this rule. This also contains phosphate of lime, sometimes to the extent of 15 or 20 per cent. There is but one bed of phosphorite; and it is very persistent in its continuity, and extends over a large area. Indeed there cannot be any doubt that wherever the Bala Limestone is found in North Wales or the borders, this deposit will be found, more or less pure, at its usual horizon.

Sometimes, however, the bed divides, mostly into two, but sometimes, as in the Berwyn Mine, into three beds. When this division takes place, the dividing substance is the thin phosphatic limestone. The uppermost bed at such times dies out as it enters the overlying shales. So also does the middle one. It is invariably the lowest bed which is continued forward, the overlying limestone dying out until the shales take their true position immediately above the phosphorite bed.

It may help to the better understanding of the whole subject if I now describe generally the strata with which the phosphorite bed is associated. It lies, as I have said, at the summit of the Bala Limestone. At Blaen Rhiwarth, where the Berwyn Mine is situated, and at a depth of about 300 feet below the phosphorite, the limestone is seen gradually passing out of the ash-bed (which close by becomes a compact greenstone) and assuming a calcareous character. Its lower beds are massive and are charged here and there with their characteristic fossils. Higher up the beds become thinner and are interstratified with shales of various textures, until at last the whole deposit is capped by the phosphorite, and shales permanently follow.

To describe the fossils of the beds below the phosphorite would be to enumerate the whole of the fossils of the Bala Limestone. I would therefore now only direct attention to two or three features in the grouping of these, which are chiefly remarkable along with the phosphorite bed by their continuity over a large area.

W.

Fig. 2.-Section of Strata at the Berwyn Phosphorite Mine, west of Llangynog, North Wales.

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a. Grey shale, with Echinoderms, phosphatized. b. Phosphate bed.

c. Dark limestone impregnated with phosphate.

d. Dark shales with veins of sulphate of baryta passing upwards into
a soapy clay.

e. Limestone, with veins of sulphate of baryta.

f. Dark shales.

9. Bluish grey limestones.

h. Limestones and shales, often pyritous, and decomposing towards the top as brownish sandstone, containing Orthis and Leptana of several species, Illanus, and Asaphus.

i. Blue slaty bed.

j. Limestones and shales, with Orthis, Leptana, Bellerophon, and other usual fossils in places.

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k. Crystalline limestone.

1. Tough blue shaly rock, with calcareous partings; fossils here and there.

m. Limestone, with Illanus, Orthis, and other fossils plentiful.

n. Beds of bluish shale, with black balls of phosphate, small Trilobites, &c.

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[blocks in formation]

q. Kaolin.

r. Alternation of slaty, calcareous, and arenaceous beds.

s. Limestone, composed almost entirely of Orthis spiriferoides. t. Shaly beds, slightly calcareous, with limestone bands. u. Superficial drift, local.

* Peat deposits.

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