98 REV. EDWIN HILL AND PROF. T. G. BONNET ON THE 



12. Ac/e of the Clastic CJiarniuood liocks. — Since the publication 

 of our papers a considerable amount of evidence, which has an 

 indirect bearing upon the question discussed in this section, has 

 been brought to light by the investigations of Professor Lapworth 

 and Mr. W. J. Harrison in the Hartshilland Dosthill district, and of 

 ^It. E. Brown in his examination of the Permian breccias of the 

 Leicestershire Coal-field. The occurrence of Cambrian rocks in 

 Warwickshire has been proved by the evidence of fossils, and these 

 overlie a quartzite of considerable thickness, under which is a 

 volcanic series. The last, however, does not, in our opinion, closely 

 correspond with the Charnwood rocks of similar origin, but appears 

 more nearly to resemble the volcanic groups beneath the quartzitcs 

 at the Lickey and in the Wrekin district. In the last-named it has 

 been demonstratefl that the quartzites cannot be newer than the 

 very lowest member of the Cambrian age*. The attempts to link 

 within that system the volcanic groups at St. David's and of the 

 Bangor-Llanberis district have not, in our opinion, been successful. 

 Hence it seems more in accordance with the usual principles of 

 geological nomenclature to separate these volcanic deposits from the 

 Cambrian, and place them all provisionally in the system for which 

 Dr. Hicks has proposed the name " Pebidian." As to the magni- 

 tude of the break between the two, it is difficult, at present, to 

 express a positive opinion ; it may not have been greater than that 

 between Ordovician and Silurian, but their physical conditions 

 appear to have been very different. The Pebidian period was 

 characterized, in the districts hitherto recognized in Britain, by 

 considerable volcanic activitj^ so that not only agglomerates and 

 ashes are abundant, but also even the finer slates suggest the 

 presence of large amounts of volcanic dust. These materials no 

 doubt were often spread out and modified by the action of water, 

 but probably the sea was shallow and interrupted hy land in the 

 above-named region. The Cambrian period, however, appears to 

 have been one of steady, continuous depression, during Avhich the 

 older land- surfaces gradually disappeared beneath the waves, so that 

 beds of a much more uniform and ordinary sedimentary character 

 were deposited. The Charnwood Group, as we formerly stated, 

 presents resemblances to the volcanic Ordovician rocks of the Lake 

 District, but the tendency of the evidence since we wrote appears to 

 us strongly in favour of referring the Porest rocks to the latest 

 epoch in the pre-Cambrian series — the Pebidian. 



V6. Age of the Ljneous Roclcs.- — The paragra2)h on this subject in 

 Part. II. is of course cancelled by our reference of the sedimentary 

 rocks of Charnwood to a Pebidian instead of an Ordovician age. 

 We are not indeed prepared with any other suggestion, but may 

 say that it seems probable that at any rate the larger masses of 

 igneous rocks were intruded prior to the earth-movements which 



* Thev are assigned by Professor Lapworth to the base of the Olenellus zone 

 ('Nature/ Dec. 27. 1888). 



