THE GEOLOGICAL RECORD 211 



ous fonns of arthropods, insects, primeval fishes and am- 

 phibians, comprises a thickness of stratified rocks somewhat 

 greater than that of the whole of the Secondary and Tertiary 

 strata combined. This thickness, which can ho measured with 

 considerable approach to accuracy, is generally supposed to 

 afford a fair 'proportionate indication of the lapse of time. 



There is a popular impression that in these remote ages the 

 forces of nature were more violent, and their results more 

 massive and more rapidly produced, than at the present time; 

 but this is not the opinion of the best geological observers. 

 The nature of the rocks, though often changed by pressure and 

 heat, is in other cases not at all different from those of subse- 

 quent ages. Many of the deposits have all the characters of 

 having been laid down in shallow water, and in several cases 

 footprints of Amphibia or reptiles have been preserved as well 

 as impressions of raindrops, so exactly corresponding with those 

 which may be seen to-day in suitable places, that we cannot 

 suppose the operations of nature to have been more violent 

 then than now. All our great coal deposits of PalaBOzoic age 

 indicate long, and often repeated, but very slow depression of 

 large areas of land, with intervening periods of almost perfect 

 stability, during which dense forests had again time to grow, 

 and to build up those vast thicknesses of vegetable matter 

 which, when buried under successive rock-strata, became com- 

 pressed into coal-seams, usually of several feet in tliickness. 



It is an extraordinary fact that in all the great continents, 

 including even South America and Australia, coal-fields are 

 more or less abundant at this period of the earth's history. 

 This is proved by the identity or close similarity of the vege- 

 tation and animal life, as well as by the position of the coal- 

 beds, in regard to the strata above and beneath them. It is 

 true that coal is also found in some Secondary and Tertiary 

 strata, but these beds are much less extensive and the coal is 

 rarely of such purity and tliickness ; while the later coal-fields 

 are never of such world-wide distribution. Tt seems certain, 

 therefore, that at this particular epoch there Avere some spe- 



