ANNIVERSARY ADDRESS OF THE PRESIDENT. 



55 



oven before the Cambrian rocks were accumulated : remnants of this 

 early land or continent are still visible and traceable in Spain, 

 France, Scandinavia, Ireland, Scotland, England, and Wales. Gneiss, 

 granitoid rocks, and occasional limestones formed the mass of this 

 Pre-Cambrian land, now exhibited to us in a greatly metamorphosed 

 state, stratigraphically unconformable to the overlying Cambrian, 

 and with a discordant strike north of latitude 30°. These rocks are 

 visible on both sides of the Atlantic as far as the Arctic regions, 

 representing portions of two continents, Europe and America, 

 now separated by the North Atlantic. 



Pre-Cambrian Rocks. 



It may be received as an axiom that all the known older sedi- 

 mentary rocks were deposited more or less in a similar manner, 

 or under similar conditions, to those of modern times. Allowing 

 this to be the case, it is most probable that those sedimentary 

 deposits which are the lowest, or have the earliest position, have 

 undergone most change ; and it must be admitted that the crystal- 

 line, semicrystalline, and metamorphosed state in which they now 

 appear has been subsequently induced through various agencies 

 (heat, pressure, and chemical change) exerted through countless 

 ages. Research seems to prove that most of the rocks now recog- 

 nized as Pre-Cambrian were originally sedimentary- strata, which 

 have undergone, since their deposition, alteration or metamorphism. 

 Eew men have paid more attention to the physical history, distri- 

 bution, succession, and character of this most ancient group of rocks 

 than Dr. Hicks, ably followed by Professor Bonney, Professor Hughes, 

 Mr. Tawney, Dr. Sorby, Professor Hull, and Mr. Hudleston ; each of 

 these observers has contributed largely to the elucidation of the 

 history of the Cambrian rocks in his own particular way. Three 

 if not four groups or systems have been determined by Hicks, having 

 definite bearings or discordant strikes one to the other through 

 certain portions of the British Islands. They occur in ascending- 

 order, and in the order of time, as follows :— 



1. Lewisiau. Hebrides and North-west Highlands. 



2. Dimetian. St. David's, Caernarvon, and Anglesey. 



3. Arvonian. Pembrokeshire, Caernarvonshire, Anglesey, Shropshire, 



and the Harlech Mountains. 



4. Pebidian. South Wales, Shropshire, Charnwood Porest, Anglesey, 



and Ireland. 



Lewisian. — Murchison gave the name Lewisian to the crystal- 

 line rocks largely entering into the structure of the Hebrides and 

 North-western Highlands ; probably these constitute the oldest group 

 of rocks known or recognized in the British Islands. Red felspar, 

 hornblende, and quartz are the prevailing ingredients in these mas- 

 sive gneisses ; occasionally, as at Cape Wrath and on the coast to the 

 south, almost pure hornblende rock occurs. The strike of the 

 Lewisian group is usually east and west, or ranging between that 

 and north-west and south-east. 



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