278 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



which are remarkably persistent as a whole, though varying from point to 

 point in the relative proportions of calcareous and silicious material enter- 

 ing into their composition, give evidence, in their even and thin beds and 

 in their fineness of grain, of a slow and uniform deposition in quiet and 

 rather deep waters. Even in the conglomerate, which is invariably found at 

 the base of the series, only very small pebbles of the very hardest and most 

 tenacious forms of quartz are found. Neither fragments of Archean -rocks 

 nor even feldspar fragments occur in them. The lower calcareous beds also, 

 in spite of their dolomitic character, are usually compact and fine grained. 



In the middle member of the Carboniferous, however, a decided change 

 in the character of the sediments takes place : they become, as a rule, very 

 coarse-grained, cany feldspar and mica and rolled pebbles of granite and 

 schist ; they often contain carbonaceous matter, which is sometimes concen- 

 trated into actual beds of coal along the borders of the original land mass, 

 and remains of plants peculiar to the Carboniferous period are found in them 

 at a considerable distance from the supposed shore line. It is evident, there- 

 fore, that in the middle Carboniferous epoch the seas became shallower, that 

 the abrasion of the land masses was more rapid than theretofore, and that on 

 the land vegetation flourished luxuriantly in this mountain region, as it did 

 at the same period in other parts of the world. 



During the succeeding Upper Carboniferous epoch and also in the 

 Mesozoic era the same coarser character of sediments prevails, although 

 carbonaceous deposits are wanting until towards the close of the Creta- 

 ceous. Both in the Weber Grits and the Upper Coal Measure formations 

 the calcareous deposits are not only very subordinate in quantity but very 

 variable ; at one point in a given thickness of rocks only a single thin bed 

 of dolomitic limestone will be found, whereas within the same horizon, at 

 another point not very far removed, several may occur. 



Dolomitic sediments. One of the most noteworthy facts developed by the 

 study of the sediments of this region is the prevalence of dolomites among 

 the calcareous deposits. All the calcareous beds below the Robinson lime- 

 stone, which was taken as the base of the Upper Coal Measures, are, with the 

 unimportant exception of a locally developed silicious limestone ia the Cam- 

 brian, true dolomites of varying purity. In the hand specimen they have 



