310 The Rev. Prof. Blake — Base of the Sedimentary Series. 



change of description. We are now told that " the general likeness 

 in the whole series to a group of volcanic rocks found in more recent 

 strata is most marked, especially to those associated with the Lower 

 Silurian," a remark which never could have been made at the close 

 of the former description. Almost the only statements that remain 

 the same are that the Dimetian is bedded, and that the Cambrian 

 conglomerates " almost invariably for the most part consist of the 

 fragments of the rocks on which they lie." 



After such a change as this it is important to remark that the 

 conclusion Mr. Hudleston arrived at was that the " Dimetian " 

 represented the hypogene condition of a group of which the 

 "Pebidian" was the surface product — a conclusion which negatived 

 any bedding in the former, or any great difference in age between 

 it and the latter. 



A year later Dr. Hicks visited the district east of St. Davids, 

 and examined the rocks at Eoche and Treffgarn, which Murchison 

 had previously described as felstones, intrusive into the Cambrian 

 rocks amongst which they occur, but which Dr. Hicks described as 

 bedded, and accepted Mr. Davies's name of Halleflinta. For this he 

 created a new group, the " Arvonian," and referred to it the quartz 

 porpyhries of St. Davids, which the year before he placed at the 

 base of the Dimetian, but now placed above it and unconformable 

 to it, without giving any definite reason for the change. 



Dr. Geikie's examination of the district in 1883 gave a new com- 

 plexion to the whole matter. He showed that the whole of the 

 '• Dimetian," as last restricted, was nothing but a granite throughout, 

 and stated also that it was intrusive into the Cambrian. In the 

 first of these conclusions he was supported by Prof. Eenard, 

 and by all subsequent observers, as he had practically already been 

 by Mr. Hudleston, but the supposed evidence of the second point 

 has not been considered sufficient for its proof. The quartz porphyries 

 which had been called " Arvonian " he considered as apophyses of 

 the granite ; and as regards the " Pebidian," while accepting the 

 later petrological descriptions of Dr. Hicks, he minimized the 

 separation between this and the Cambrian conglomerate, and 

 challenged Dr. Hicks to prove his statement that the latter is chiefly 

 composed of " fragments of the rock on which it lies." This Dr. 

 Hicks has never been able to do, seeing that the most marked feature 

 of the conglomerate is that it is not composed of such fragments ; 

 instead of doing so, he now attempts to show that the matrix might 

 have been derived from granitoid rocks. 



My own observations, while coinciding with the general ideas of 

 Dr. Geikie, and differing chiefly on the question of how far the 

 granite intrudes, led me to think the gap between Cambrian and 

 Pebidian a wider one than he admits. Eecently Prof. Lloyd Morgan, 

 going over the same ground, brings them closer together again. 



As to the " Arvonian " in its typical locality, I have shown that 

 the supposed bedded halleflinta at Koche is nothing but a silicified 

 andesite, and this determination has been accepted by Prof. Bonney 

 and others and thus — exit Arvonian. 



