378 Correspondence — Mr. J. E. Marr. 



a felsite. The Eev. J. F. Blake denied this. He admitted that 

 " some eminent petrologists " had, on microscopic examination, 

 pronounced the rock from which the gneiss has been formed to 

 be a felsite ; but he would not yield to their testimony. He had 

 seen the rock in the field, and he " regretted " that these authorities 

 bad made a mistake. It may seem a little presumptuous for Mr. 

 Blake to contest a determination made by three of the best petro- 

 graphers in Europe ; and we may suspect that if Ajas ventures to 

 defy the lightning, Ajax will not do himself much good. Most 

 people will consider that such an authoritative identification is 

 absolutely decisive, and I, for one, assume it to be so. My main 

 purpose in writing is, however, to point out that even the field- 

 evidence, which, in Mr. Blake's opinion, refutes the " eminent 

 petrologists," is dead against him. Within a hundred yards of 

 the critical section at Y G-raig, in which the felsite passes into the 

 gneiss, a rock which closely resembles the felsite and which also 

 graduates into a similar gneiss, is seen to penetrate the diorite, 

 which is more or less altered, in numerous veins, some of which 

 are branched. Thus the microscope of the "eminent petrologists" 

 and the eyes of the field-observer who knows where to look, lead 

 to the same conclusion. Mr. Blake himself admits that the rock 

 thus shown to be a felsite " passes by insensible gradations into the 

 ordinary grey gneiss." He says that the " phenomenon is a purely 

 local one." This is a mistake, as I can prove by examples to the 

 contrary, though the felsitic structure is nowhere, I believe, so 

 marked as at Y Graig ; but if it were so, my case is not materially 

 afiected. If felsite is converted into a typical gneiss at Y Graig, all 

 primd-facie objection to such a change disappears, and the field- 

 evidence receives important confirmation. 0. Callaway. 

 Tewkesbury. 



Mai/ 11, 1897. 



TRINUCLEU8 SETICORNIS AND THE UPPEE BALA. BEDS. 



Sir, — In the report of a discussion of a paper on the fauna of the 

 Keisley Limestone, Part II (Q J.G.S., vol. liii, p. 106), the author 

 is stated to have made the following remark : " Mr. Marr .... 

 denied that Trinucleus seticornis was specially characteristic of Upper 

 Bala beds, though some years ago he had called the Upper Bala beds 

 of the Haverfordwest area after that fossil." I should hardly have 

 supposed that my words requii-ed an explanation, but as the speaker 

 on that occasion appeared puzzled, I will give it. 



Mr. T. Eoberts and I (in 1885) called the Upper Bala beds of 

 Haverfordwest Trinucleus seticornis beds, because the fossil or one of 

 its varieties is particularly abundant in those beds in that area, and 

 we did not there discover it in beds of Middle Bala (Caradoc) age. 

 We were perfectly justified in our action, and there is precedent for 

 it ; e.g., no one supposes that the Leda myalis beds of the Cromer 

 Forest Group are the only beds with Leda myalis. If we had spoken 

 of the beds as constituting the zone of Trinucleus seticornis an 

 explanation might be necessary, although I hold that even that 



