MANNER OF OCCUBBENCE OF DEPOSITS. 541 



the Blue or Lower Carboniferous Limestone. It is true that these deposits 

 in the western United States are mainly confined to Paleozoic beds, but 

 this is hardly to be wondered at when one considers that the other hori- 

 zons are almost entirely silicious or argillaceous and contain as a rule a 

 very limited development of limestone. It is evident that broad general- 

 izations on the basis of geological horizon alone are not only unfounded 

 but are misleading, and that it is more logical to seek for an explanation 

 in the local conditions of each individual district. 



The causes that may have influenced the concentration of ore in any 

 particular bed of limestone may have been physical or chemical ; that is, 

 the structural or physical conditions of the region may have been such that 

 the solutions were naturally directed to that particular bed, or the composi- 

 tion of that bed may have been such as to render it peculiarly susceptible 

 to the action of waters reaching it from adjoining rocks or to cause the 

 precipitation of the minerals held in solution by those waters. 



The physical or structural conditions of this region have been shown 

 by geological descriptions to be peculiarly favorable to the concentration 

 of percolating waters in the Blue Limestone. The great intrusive sheets 

 of porphyry are found to follow it most persistently, mainly along the 

 upper surface, less frequently along its under surface, and also cutting 

 transversely across it. These intrusive bodies are also found at other hori- 

 zons, it is true, but at none so persistently and so uniformly as at this. Thus 

 both ascending and descending currents would readily reach these beds, the 

 latter trickling through the uniformly permeable eruptive rock, the former 

 following up the walls of the channel through which it was erupted Such 

 waters, after passing through a medium of different composition, would be 

 ready for a chemical interchange with the limestone, but in the case of 

 ascending waters it does not appear evident why this interchange should 

 have taken place along one wall of the channel rather than the other, while 

 with descending waters this action would naturally commence on the upper 

 surface of the limestone bed. Thus the physical conditions afford a reason 

 for the predominant choice of this horizon. 



As regards the chemical composition of the bed, the evidence is less 

 conclusive. Some authors have been inclined to regard dolomitic limestone 



