:he teiassic breccias and costglomerates of south devox. 69 



7. The Igneotjs Constituents of the Triassic Breccias and Con- 

 glomerates of South Devon. By R. N. Worth, Esq., F.G.S. 

 (Eead December 4, 1889.) 



In the paper on " The Elvans and Yolcanic Eocks of Dartmoor " 

 which I had the honour of laying before this Society on the 3rd of 

 April last, I mentioned that the examination of a number of 

 fragments of igneous rocks included in the Triassic breccias and 

 conglomerates of South Devon had shown that many so-called 

 *' porphyritic traps " among them were really " elvans " or felsites, 

 whilst others presented a near approach to true volcanic types *. 



My present purpose is to follow up that remark by giving the 

 results of a detailed investigation of the igneous constituents of 

 these beds. The subject is by no means a new one ; for the origin 

 of these fragments has now and again engaged the attention of 

 geologists, some of great eminence, for nearly seventy years. But 

 the opinions expressed have been diverse, and the problem can 

 neither be regarded as settled nor as having approached a definite 

 solution. So far as I am aware, indeed, no recent attempt has been 

 made at a general investigation ; and there has certainly been no 

 such application of modern methods of petrological research. 



Under these circumstances it is hoped that the present paper may 

 prove of service as a record of observed facts affording material for 

 judgment -, and that it will not be thought the attention of the 

 Society has been needlessly called to a topic in itself perhaps unim- 

 portant, but which, nevertheless, has weighty bearings on the 

 physical history of the West of England. There is no occasion, for 

 example, to enlarge on the proposition that very different conclusions 

 would have to be drawn with regard to the conditions prevailing 

 during the formation of these breccias and conglomerates, from the 

 presence in them of foreign rocks on the one hand or of native rocks 

 on the other. 



I preface the results of my own inquiries with an outline sketch 

 of the 



History op the Question. 



The first detailed attempt at a lithological examination of these 

 rocks was made by the Rev. J. J. Conybeare, F.R.S., and set forth 

 in a letter in the ' Annals of Philosophy,' dated February 1821t. 

 Mr. Conybeare identified " Granite and Porphyritic Bocks " as 

 forming a " very considerable portion of the imbedded fragments ;'* 

 and described them under five classes and twelve varieties, in terms 

 which show that he had before him just such rocks as are com- 

 monly present now, specially indicating schorlaceous pegmatites and 

 " elvans," and suggesting " that the whole contents of this breccia 



* Quart. Journ. Geol. Soc. vol. xlv. p. 391). 

 t 2ud ser. vol. i. pp. 254-259. 



