516 Dr. H. Hicks — Pre- Cambrian Rocks in Conglomerates. 



A full account of the history and literature of the question of the 

 underground range of the older rocks in the South East of England, 

 especially as regards the possible occurrence of Coal Measures, has 

 been given in lately published Geological Survey Memoir, " The 

 Geology of London and of Part of the Thames Valley." 



VI. — On Pre-Cambrian Eocks Occurring as Fragments in the 

 Cambrian Conglomerates in Britain. 



By Henry Hicks, M.D., F.R.S., F.G.S. 

 (Read at British Association, 1890.) 



^F late years the Cambrian Conglomerates have received a con- 

 siderable amount of attention, as it has been shown that much 

 information concerning the nature and condition of the Pre-Cambrian 

 rocks can be obtained by the careful examination of the materials 

 composing the conglomerates in different areas. It is evident that 

 rocks similar to the rolled fragments in the Conglomerates must 

 have been present in pre-Cambrian times to yield these fragments, 

 and if in certain areas special rocks occur abundantly or otherwise, 

 the characters of the pre-Cambrian rocks in these areas may be to 

 a great extent determined. The Cambrian Conglomerates in Wales 

 and Shropshire have been very carefully examined by Prof. Hughes, 

 Dr. Callaway, and myself, and the materials collected have been 

 submitted to Prof. Bonney and Mr. T. Davies for microscopical 

 examination. These eminent penologists have from time to time 

 described the results of their examinations and have shown most 

 conclusively that certain fragments which occur in the Conglomerates 

 are in the minutest particulars identical with rocks which have been 

 claimed by us, in the areas in which the Conglomerates occur, as 

 being of pre-Cambrian age. In some areas the Conglomerates have 

 been found to be composed almost entirely of materials from rocks, 

 which can clearly be shown to underlie them, and the special 

 characters recognizable in the rocks below are found in the rolled 

 fragments in the Conglomerates which rest unconformably upon 

 them. I have endeavoured to indicate by a table the contents of 

 the basal Cambrian Conglomerates in Britain in several of the areas 

 where it has been claimed that pre-Cambrian rocks are now exposed, 

 and an analysis of this table is in many ways very suggestive. It 

 is found that certain rocks occur in each of the areas, whilst others 

 are limited to those districts where special rocks are known to charac- 

 terize the pre-Cambrian series. In the first column 1 have given 

 the rocks which I have collected in Pembrokeshire, and these are, 

 in the main, very similar to those which have been collected in 

 N. Wales and Shropshire. Those mentioned from Boss and Suther- 

 land have either been collected by myself, or are mentioned in 

 papers by the officers of the Geological Survey. The Cambrian 

 Conglomerates near St. David's, Pembrokeshire, have perhaps 

 received more attention than those of any other area, and the 

 notes furnished by Prof. Bonney and Mr. T. Davies prove in the 



