STRATIGRAPHICAL GEOLOGY. 17 



Among the Cephalopoda, numereus genera and species had disappeared. 

 The Nautilus the highest form then developed, was common, and furnishes us with 

 several subgeneric types, thus manifesting its prosperitj^ and pointing to its con- 

 tinuance in succeeding strata; while the Orthoceras had become rare and diminu- 

 tive, preparatory to interring the last of the family in this Group of rocks. 



Among the Gasteropoda there was a decided advancement from the marine 

 forms, to the land snails of the genus Pupa. 



Among the Articulates the progress in animal life was still more clearly 

 manifested, not only in the appearance of the marine forms of the genera Eu- 

 proops, Acanthotelson, and Anthrapalcemon, but in the appearance of terrestrial 

 insects of the family iVe(trop<er»d<B; Myriapods of the genera Hoscorpitcs, Ma- 

 zonia and Architarbus; while the Trilobites that swarmed in frail hulks in earlier 

 days gradually became extinct. 



The wheel of evolution rolled yet more rapidly among the Vertebrata. The 

 fish became more diversified and more highly organized. Amphibian animals 

 made their appearance in several families, some of them were protected by scales, 

 others were not; some had long vertebral columns, others had short ones; some 

 had limbs well developed, and in form were lizard-like, while others were desti- 

 tute of limbs or possessed them weak and half developed. 



The Permian Group was so named by Murchison from Perm, a government 

 in Russia. This Group is known only in the country West of the Mississippi, 

 and is so intimately connected with the Coal Measures as to be hardly separable. 

 Its maximum thickness does not exceed 500 feet. The maximum thickness, 

 therefore, of the Carboniferous Groups may be placed at 24,100 feet. 



The Carboniferous rocks, as found in the Uinta Mountain Region, have been 

 subdivided by Powell and others, into four Groups, viz. : Lodore Group, Red Wall 

 Group, Lower Aubrey Group and Upper Aubrey Group. 



The maximum thickness of the Groups forming the PaliBozoic rocks of 

 North America, as here shown; is as follows : 



Laurentian 32,750 feet. 



Huronian 20,000 " 



Lower Silurian 48,745 " 



Upper Silurian 8,000 " 



Devonian 15,235 " 



Carboniferous 24,100 " 



Total 148,8.30 feet. 



This is a little over 28 miles, all of which is known to be fossiliferous, except 

 the three miles at the base. It may be, that the thickness of some of the Groups 

 is overestimated, and it may be, that two Groups, which are estimated, were de- 

 posited at the same time, and that only one of them should be counted ; but on 

 the other hand, it may be, that some Groups are entirely omitted, and that others 

 have not been measured at the place of their greatest thickness. The probability 

 is, therefore, that the maximum thickness of the Groups, when more certainly as- 

 certained, as it will be by future explorations and measurements, will not fall much, 

 if at all, below the present estimate. 



The limestones of the Palaeozoic rocks were formed in clear ocean waters, 

 from the remains of calcareous shells. Their formation must have been ex- 



