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sons TRILOBITES FRO:!<I THE SKIDDAW SLATES. 469 Nicholson did not feel sure of its correct identification, it has been deemed better to notice it only by an outline figure. (J. G. G.) The trilobite fauna of the Skiddaw Slates, then, so far, may be stated as follows :- Phacops nicholsoni, Salter. Cybele ovata, Etheridge. ? Calymene breoicapitata, ? Iicmopleurides sp. ? Olenus sp. ? Tl'inucleus gibbsii. New genus, represented by No. 1. ? Ogygia Sclwgni. ? Ogygia sp. Barratulea falcata, n.s., P. & G. Niobe doveri, Eth. Eurmnetopus cumbrianus, n.g. & n.sp., P. & G. Eurymetopus horrisonii, n.g. & n.sp., P. & G. .diglina? redioioa, Barr• .diglin a ? binodosa . .JEglina obtusicaudata, Hicks. LEglina? caliginosa. Aqnostus morei, Salt. OBSERVATIONS UPON THE STRATIGRAPHICAL RELATIONS OF THE SKIDDAW* SLATES. By J. G. GOODCHILD, H.M. Geol. Survey, F.G.S., F.Z.S. From the days of Jonathan Otley down to within the last few years, Otley's views in regard to the geological position of the Skiddaw Slates in relation to the other rocks of the Lake District have been accepted unquestioned by geologists. Of late, however, chiefly since the results of Dr. Hicks's researches amongst the Ar­ chsean Rocks have been made known, many geologists seem to have been struck with the remarkably close resemblance in lithological character between the " Green Slates and Porphyries" of the Lake District and the Pebidian Rocks of the St. David's area and else­ where. They have thence been led to inquire whether there might not be something more than a mere resemblance between the two * In deference to established usage I here adopt the customary spelling of Bkiddaw ; otherwise I should prefer to write Skidda---J, G. G. 470 J. G. GOODCHILD ON SKIDDAW SLATES. great groups of volcanic rocks, and whether, after all, the Volcanic Series of Borrowdale might not represent in time those oldest known volcanic rocks of the St. David's area made known to us as such by Dr. Hicks. In discussing this question in connection with the Lake District, it was pointed out that, on the one hand, the junction between the Coniston Limestone (Bala) and the Borrowdale Volcanic Series is marked locally by a very considerable discordancy in the strike of the two rocks, so that, within short distances, the Coniston Lime­ stone could be seen lying across the ends of various and widely separated members of the older strata. Similarly, at the base of the Volcanic Series, everyone secms to have been much impressed with the fact that the junction with the older beds nearly everywhere coincides with faults, which have un­ known amounts of throw. Under these circumstances there is little wonder that geologists have felt considerable doubt as to whether the views generally re­ ceived regarding the relation of the Borrowdale Volcanic rocks to the Skiddaw Slates should continue to be accepted. Now that additional palreontological evidence has been laid be­ fore the public," it may be as well to review some of thc facts, with a view to a final settlement of at least some of the questions at issue. First, with regard to the question whether either the Skiddaw Slates or the Borrowdale Volcanic Rocks, or both, might not be inverted-the Skiddaw Slate in such a case being newer, instead of older, than the Volcanic Rocks. In regard to the Skiddaw Slate, at all events, there is not much difficulty in finding an answer to this question, for ripple-marks are common enough on all horizons throughout this vast pile of old sedimentary accumulations, and these, as is well known, offer us a ready means whereby one may determine whether the strata bearing them are inverted or not. Now, in making several traverses of considerable length across the outcrop of the Skiddaw Slates, both in the neighbourhood of Kes­ wick and in other parts of the district adjoining, I have examined these ripple-marked surfaces here and there at sufficiently-close intervals to determine the point in question. There are many small and purely local disturbances and inversions of the strata here, as there are in nearly all districts composed of rocks having a similar * Seethe paper on the" ~'rilobites of the Skiddaw Slates," preceding this. J. G. GOODCHILD ON SKIDDAW SLATES. 471 history to the Skiddaw Slates; but ignoring these (and I have only met with two or three cases of the kind), I have found it the rule that the ripple-marks remain the original way up. In all cases the crest, or hig her part of the ripple-mark, is flanked on each side by thecustomary shallow trough intervening between the crests; and this trough has the concave side, and not the convex, upper­ most, as would have been the case had a cast of the ripple-mark been under notice. With such evidence as this before us, it is easy enough to demonstrate, on the ground, that at least tlie greater part-practically the whole-of the Skiddaw Slate remains un­ inverted. Then, with regard to the 'Tolcanic Rocks. As these are pro­ bably wholly subaerial in origin, at least in the Lake District proper, the evidence afforded by ripple-marks fails us entirely. But there remain other means of testing their position. Amongst the numerous beds of lava interspersed here and there throughout the series, are many that yet retain more or less evident indications of their original vesicular and scoriaceous character. In such cases it is evident that we have means of determining which was origi­ nally the upper, and which the lower surface, by comparing the difference in extent of the scoriaceous part; referring the less scoriaceous surface to the original under-surface of the lava-flow, and the more scoriaceous and vesicular zone to the original upper surface. Applying this test to the volcanic strata of the Lake District, we find that, as a rule, the more vesicular surfaces are still uppermost, and we are thus able to prove conclusively that these rocks also are not inverted, but are simply inclined from their original position, a variable, but never very great,number of degrees. This inclination, by the way, is usually much less in amount than either the average angle of dip of the Skiddaw Slates, on the one hand, or than that of the Silurian Rocks on the south side of the Lake District on the other. The reason why this is so will appear on further consideration presently. The point remaining to be considered, then, is whether any clear case can be made out where the Voleanic Rocks alternate with Skiddaw Slate. Mr. Ward has himself shown that this is the case at several isolated localities along the line of junction of the two formations. Volcanic Rocks interbedded with the Skiddaw Slates may be seen near Hollas, close to Grange, not far below the sec­ tion pointed out by Mr. Postlethwaite, at the Keswick meeting of VOL. IX., No.7. 34 472 J. G. GOODCHILD ON SKIDDAW SLATES. the Cumberland and Westmoreland Association (May, 1886), as a junction between the Borrowdale Hocks and the Skiddaw Slate. Outside the Lake District proper the contemporaneity of some thousands of feet of volcanic strata with the Skiddaw Slates is well seen in the sections near Milburn, which were described many years ago in the' Quarterly Journal of the Geological Society' by the late Prof. Harkness. These last-named sections alone, to my mind, are sufficiently conclusive in regard to the contemporaneity, or, rather, to the stratigraphical continuity, of the two deposits. They suggest that the Skiddaw Slates represent the normal type of accumulation prevailing all over this area in the rocks below the Coniston Limestone, and that the volcanic pile now seen in the Lake District represents, as one might expect to find it, merely a local and exceptional episode in the history of a great accumula­ tion of strata otherwise mainly of sedimentary origin. The whole of the evidence simply points to the conclusion that the Borrowdale type of strata represents one or more great piles of volcanic material accumulated, during the later part of the Skid­ daw Slate period, at such a rate, around the foci of eruption, as to have prevented the deposition of any marine sediments there; while on the immediate outskirts of the volcanic area volcanic ejectmenta were interbedded with the sedimentary strata, or were intermingled with them in varying proportions, which diminished with increased distance from the centres of erup­ tion. The volcanic rocks thus shaded off all round into sedimen­ tary deposits pure and simple. The Milburn Rocks, first described in detail by Prof. Harkness, represent one of the intermediate types of deposit, accumulated beneath the sea, on the outskirts of the old volcanic area. Some other difficulties connected with the relation of the Volcanic Series to the Skiddaw Slates will be discussed further on. But let us now examine some further stratigraphical evidence bearing upon the relation of these rocks to strata whose place in the geological scale can be fixed with approximate certainty. Up to the time when Prof. Harkness wrote his paper in the' Quart. Journ. Geol. Soc.,'* hardly any attempt had been made at fixing, even ap­ proximately, the age of the great group of strata under notice. In that year he showed that on the south side of the Lake Dis­ trict, in the neighbourhood of Kentmere, and from there to * 'Q. J. G. S.' xxii, p, 480. J. G. GOODCHILD ON SKIDDAW SLATES. 473 Shap Wells, the Coniston Limestone, whose fossils clearly proved it to be the paleeontological equivalent of the Bala Lime­ stone of North vVales, conformably overlay a thick mass of vol­ canic rocks, which, in turn, graduated downwards into a second calcareous horizon, containing other fossils, also clearly of Bala types.

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