32 PAL.^OZOIC. 



essentially a fundamental and primitive rock is so far modified 

 that we recognize its intrusion from depths below at various 

 epochs in the earth's history. Nevertheless, its formation, so 

 far as we can read it, was always deep-seated ; some granites 

 are among the oldest rocks, and the date of the upthrust of 

 others is indicated by the alteration produced on adjoining 

 strata. 



In the Primary or Palaeozoic group we may include the 

 earliest known rocks (Archaean ^) and those formed up to the 

 close of our Coal period. Some authorities still place the 

 Permian rocks in this group, but in their sedimentary cha- 

 racter and method of formation, they are so closely allied to 

 the Triassic strata, that these two divisions are best classed 

 together to form the New Red Sandstone or Poikilitic system 

 at the base of the Secondary or Mesozoic group. The follow- 

 ing are the Palaeozoic systems : — 



Carboniferous. 



Old Red Sandstone and Devonian. 



Silurian. 



Cambrian. 



Archaean. 



The total thickness of the Palaeozoic group may be roughly 

 estimated at 70,000 feet, whereas the thickness of all the 

 newer strata in England and Wales probably does not 

 exceed 14,000 feet. And it is well to bear in mind what a 

 vast series of strata these older rocks comprise, for they do 

 not enter so largely as do the Secondary or Quaternary rocks 

 into the superficial structure of the land. In the lower 

 divisions of the Palaeozoic rocks the rarity and frequent 

 obscurity of the organic remains, the effects of igneous in- 

 trusion and metamorphism, and the faults and contortions, 

 add many difficulties to the study of the strata. The Carbon- 

 iferous rocks, indeed, are readily recognized, but the exact age 

 of the Devonian strata is a subject still under discussion. 



The Palaeozoic rocks generally possess a more crystalline 

 and slaty structure than the newer strata ; they occupy more 

 elevated ground, forming the lofty hills and mountains in 

 Cumberland and Wales, the Pennine Chain, the Malvern 

 Hills, and the greater part of Devon and Cornwall. The 

 older rocks, especially, furnish a poor soil, the beds often 



1 Sedgwick originally proposed the term Protozoic for the Pre-Cambrian rocks 

 (Proc. G. S. ii. 684) ; while the term Azoic has also been applied to the older 

 crystalUne and unfossiliferous rocks ; but neither term is much used. The term 

 Eozoic has also been employed for the oldest rocks that contain organic remains. 



