TRANSACTIONS OP THE SECTIONS. 107 



Oil tJie Camhrian and Sihtrian Rods of Ramseij IshmJ, St. Dav'uVs. 

 Btj Hexry Hicks, F.G.S. 



In a Eeport to the British Association iu 18GG by the hite Mr. Salter and the 

 author, Eamsey Iskud was mentioned as a part of the district which had been • 

 examined, and a short description of the rocks exposed there was also given. Since 

 that time the author has had several opportunities of further examininfy the island, 

 and this year iu conjunction with Messrs. llomfray, Lightbody, Kirshaw, and 

 Iloplrinsou. 



During- these researches all parts of the strata have been very carefully exa- 

 mined ; and the results have been highly satisfactory. The best section occurs at 

 the north end of the island ; and the following formatioiis occur tJiere iu succession, 

 bogiuning on the east side : — 



1. Liiu/uIa-Jiag^, a series of hard siliceous sandstones, with grey flaky slate, about 

 GOO feet in thickness, and containing LhujuMla JDavisii in great abundance, but no 

 other fossils, save worm-tracks and burrows, and some plant-like niarkino-s. 



2. Tremadoc group, or, rather, a thick series of beds holding in the succession 

 relatively the same position as the Tremadoc rocks do in North Wales. These 

 beds graduate by almost insensible degrees from the Lingula-flags, first as bluish- 

 grey ilag, and then earthy grey thick-bedded I'ock, and assume at the upper end an 

 appearance approaching to that of the overlying beds of the Arenig group. They 

 have a thickness of frona 800 to 1000 feet; and fossils are very abundant through- 

 out the whole series. _ The species are nearly all new, and also' many of the genera. 

 A list of the fossils includes four species of Brachiopods, ten species of Trilobitos, 

 Orthoceras two species, Ctenodonta two species, a Thcca, Bclicrophon, Encrinite, 

 and aStarfish, the latter discovered for the first time this year by Mr. Lightbody. 

 In_ this fauna, as in the Tremadoc rocks of North Wales, some of the forms are 

 primordial in character, others of a Silurian type ; and there are several which had 

 not previously been known to exist in rocks of so early an age. With the ex- 

 ception of the rocks in the neighbourhoods of Portmadoc and Dolgelly, we do not 

 know of any deposits of the same age in Britain ; and, indeed, until the discovery 

 of these beds at Ramsey Island, and some other places in the neighbourhood of St. 

 David's, they were not supposed to extend beyond those districts. 



3. Arenig group. — Iron-stained slates and flags, interlined by felspar lines and 

 felspathic ashes. They have a thickness of about 1000 feet, and lie nearly vertical. 

 They occur in succession to the Tremadoc group, and in true conformability. Tri- 

 lobites of the genera AsapJms, Ogggia, jEgliiia, Trinucleiis, Amptj.r, Ccdymene, and 

 Agnostus occur in them along witli Cumdaria, BcUcnyjyhon, Thcca, Orthoceras, Lin" 

 gula, and Ortltis, and, as shown by Mr. Ilopkinson, also no less than twenty-two 

 species of Graptolites of various forms. 



In this section at Eamsey Island the succession from the Cambrian rocks to the 

 Silurian is, the author believes, better shown than at any other known place iu the 

 British Isles. ^ 



On the Graptolites of the Arenir/ Roch of St. David's. 

 By John Hopkinson, F.O.8., F.R.M.S. 



In the lowest beds of the Silurian rocks at St. Da^•id's the author had found a 

 considerable number of Graptolites, which, he thought, proved the equivalency of 

 these beds with the Quebec group of Canada, the Skiddaw slates of Cumberland, 

 and the Ai-enig rocks of Shelve. 



The Graptolites, of which more than twenty species have been determined, were 

 found at Ramsay Island and Whitesand Bay, in the lower part of a series of black 

 shales about 1000 feet in thickness, which, from their position and from the evi- 

 dence afforded by the fossils they had previously yielded, had already been infen-ed 

 to be of Arenig age. 



Of the true Graptolites or Rhabdophora the genera Didgmngrapsm, Tdragrapsus, 

 Liiganograpisiis (?), and Phi/lhgrapsus occur. All are Quebec and Skiddaw forma, 

 Didgmograpsns alone passing into higher rocks. The Graptolito allies are repre- 

 sented by the genera JPtilograpstts, Ucndrograpiits, Calhgraptus, and Dictyonema. 



