n. HICKS ON THE PRE-CAMBRIAN ROCKS OE ST. DAVID'S. 239 



into the Cambrian rocks, the latter really rest upon and have been 

 for the most part derived from them. One mass in Ramsey Island, 

 which was supposed even to have been intruded into the Arenig 

 rocks, consists only of these altered beds with the Arenig rocks 

 brought down against it by enormous faults. 



In these metamorphosed rocks are found two distinct formations 

 unconformable to one another. Moreover the rocks belonging to 

 these formations not only show clear evidence of having undergone 

 the several changes of depression, elevation, and metamorphism, 

 previous to the deposition of the Cambrian rocks, but also that the 

 one had undergone all these changes before the deposition of the 

 other. It is evident, therefore, that these Dimetian and Pebidian 

 rocks formed portions of islands or continental lands in Pre- 

 Cambnan times, and that they were subsequently depressed to 

 receive the sediments now known to us under the name of Cambrian. 



The various and frequent changes to which these rocks must have 

 been subjected since they were deposited, and which produced the 

 very high state of metamorphism in which they are now found, 

 would doubtless also be more than sufficient to have removed all 

 traces of any organisms that may have been present in them at first. 

 Therefore the absence of fossil evidence, when the high state of 

 metamorphism is taken into consideration, does not in any way tend 

 to the conclusion that the seas or continents during the early epochs 

 represented by these rocks contained no animal or vegetable life. 

 Indeed it would be most unnatural if such were the case ; for the 

 mode of deposition and character of the sediments indicate suitable 

 marine conditions, and also that there was nothing in the chemical 

 character or heat of the water which could be detrimental to life. 

 Moreover the limestones, and the large proportion of carbonate of 

 lime found in some of the indurated shales and quartz schists, seem 

 strong evidence of the presence of life at the time they were de- 

 posited — indeed, to my mind, are sufficient proof of there having 

 been the direct intervention of life itself. 



As far as can at present be made out, these rocks seem to hold 

 the same position in regard to the Cambrian series as do the 

 Laurentian rocks in Canada ; and there are many points of resem- 

 blance in the rocks themselves. The Malvern rocks described by 

 Dr. Holl, and some altered beds mentioned by Mr. Maw as under- 

 lying unconformably the Cambrians in the Llanberis slate quarries, 

 probably belong to one or other of these series, as also the gneissose 

 rocks in the west of Scotland and in Norway, Sweden, &c. Still, as 

 there is an entire absence of fossil evidence, and frequently of suc- 

 cessional evidence also in some of these places, I feel at present that 

 any attempt at a general correlation would be premature. 



EXPLANATION OF PLATE X. 



Map and Sections of Pre-Cambrian Rocks near St, David's. 



