124 GEOLOGY AND PALAEONTOLOGY. 



the most conspicuous rock of the north and south ranges in the vicinity of the Great Salt Lake. 

 The limestone from these localities is a dark-blue, compact rock, often, in specimens, gray and 

 subcrystalline, and sometimes with sparry veins. 



This limestone, identifed by its fossils, is found in the collections made near Santa Fe^ New- 

 Mexico, at the Pecos village, the Mogollan Mountains, at El Paso, upon the river San Pedro, 

 and the numerous other localities. From a sketch and the notes of Dr. Parry, before alluded 

 to, this limestone forms a conspicuous part of some of the mountains along the boundary line. 

 There can be little doubt that the same limestone occurs much further to the southward, and 

 that it constitutes the principal calcareous formation of that part of the country, whether in an 

 unaltered or in a metamorphic condition. It is, as already stated, the limestone traversed by 

 veins of silver lead, from which specimens have been brought in the boundary collection. 



The collections of Dr. Roemer, from Texas, indicate very clearly the occurrence of this lime- 

 stone at the places examined by him ; and he gives no figures of species belonging to lower 

 carboniferous limestones. 



From the massiveness and compact texture of many of the specimens from the southwest, and 

 the subcrystalline character of others, we are prepared to find that this rock has become much 

 more extensively developed than in the northeast, or even in Missouri or Kansas ; and it would 

 appear, also, that the shaly beds which accompany the limestone in these localities and are 

 often more conspicuous than the limestone itself, have diminished so far as to form no striking 

 feature in the far west and southwest. We are not able to learn that it is there ever accompa- 

 nied by coal, and it is presumed that the shaly and sandy materials associated with the coal, as 

 well as the coal itself, have thinned out in that direction, or become of such tenuity as to be of 

 no importance. 



The relations of this limestone to the lower coal measures, in the States bordering the Missis- 

 sippi river, render its occurrence a subject of interesting economical inquiry. Since we know 

 that the most extensive and valuable beds of coal in the west are of the lower coal measures 

 which lie beneath the upper limestone, they may still be found to underlie the upper carboniferous 

 limestone of the Rocky Mountains, as they do the same limestones in Kansas, Missouri, Iowa, 

 Illinois, and Indiana. Thus far I am not aware that any inquiries of this kind have been 

 instituted in the explorations and surveys already made. 



Having thus briefly described the range of the upper carboniferous limestone, we may now 

 take a comprehensive view of its conditions and extent. We find that during the coal period, 

 in the States east of the Mississijjpi river, thin strata of limestone, or calcareous shale, 

 were deposited. These are charged with brachiopoda, of genera characterizing the car- 

 boniferous limestone below the coal measures, though of species distinct and peculiar. 

 So thin and insignificant is this formation, that we can scarcely regard it as the product of a wide 

 and deep ocean. Tracing it westward, however, its importance increases ; from being entirely 

 subordinate to the coal measures proper, and scarcely affecting the character of that formation, 

 it becomes a characteristic mass ; the calcareous mud mingles with the coal, and the latter 

 becomes subordinate to the limestone and calcareous shales. Still farther west it is a vast lime- 

 stone formation, next in importance to the great calcareous formation below the coal, or lower 

 carboniferous limestone formation. 



Tiie conditions favorable to the production of an extensive deposit of marine limestone are 



