GENERAL INFERENCES. 81 



The solution of the difficulty will not be advanced by a con- 

 sideration of the remarkable contrasts exhibited throughout the 

 series of the deposit, inclusive of the lower and the upper beds 

 of Caithness and Dura Den, where the more characteiistic 

 features of Animated Nature in the ancient period present the 

 most striking resemblances to the existing condition of things, 

 terrestrial and marine. Thus there are the large lobster-like 

 forms of Balruddery in contrast with the small shrimps of Res- 

 wallie, the huge Stagonolepis of Elgin to confront with the 

 crab-like pterichthyan fossils of Carraylie and Dura Den, the 

 mail-clad massive holoptychius of Clashbennie with the kampe- 

 caris or caterpillar-like appearance, occurring in shoals in the 

 Forfarshire flagstones, and the great cyclopteris or tree-fern, so 

 abundant in the system in Ireland, with the small berry-shaped 

 impressions of Parkhill and Tealing. 



5. Have we any means of ascertaining the amount of Time 

 that may have lapsed during the accumulation of the materials 

 of the Old lied Sandstone series, or of any of the geological 

 formations ? The ' question has man^^ aspects and bearings, 

 and there are many agencies concerned in the solution of the 

 problem. 



Looking at the current operations of the laws of Nature, 

 and supposing their uniformity in past ages, a scale of incre- 

 ment is laid down for the several deposits of which the whole 

 crust of the earth is composed. An approximation is thus 

 attempted as to the number of years required for each, and 

 the result is, that the geological estimate embraces an incon- 

 ceivably lengthened and bewildering series. The calculation 

 proceeds not by hundreds, or even thousands, but by millions 

 of the terms of our numerical notation ; and as the fossiliferous 

 strata alone are reckoned at an average of ten miles in thick- 

 ness, the time that has elapsed since the appearance of life 

 upon the planet has also been made a subject of investiga- 

 tion, including myi-iads of the brief fleeting years of man's 

 existence. 



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