22 S. F. EMMONS — THEORIES OF ORE DEPOSITION 



an advantage over that of indefinite depth appealed to by the ascension- 

 ist or hydrothermal school, inasmuch as it admitted some sort of experi- 

 mental proof, indirect though it was, and because at the depth at which 

 the rocks might be supposed to be essentially richer in metals than those 

 found at the surface, cracks sufficiently open to admit a free flow of ther- 

 mal waters were considered impossible under the conditions of pressure 

 assumed to exist there. This view was called a lateral secretion theory, 

 though it differed essentiall}'- from that of Sandberger, in that the deriva- 

 tion of the vein minerals was not restricted to the immediate wall rocks 

 (Nebengesteine) of the deposits. Indeed, in a later discussion it was 

 characterized as another form of the ascension theory. The circulating 

 waters which had brought in the vein materials were assumed, though 

 not always explicitly, to be of meteoric origin — waters which originally 

 descending from the surface had become heated either in contact with 

 igneous rocks, or by the internal heat of the earth, and gathering up min- 

 eral matter in their journey had redeposited it when conditions favored 

 precipitation rather than solution. The natural channels through which 

 these waters would circulate most freely, and which hence were most 

 favorable to ore deposition, were rock fractures produced by dynamic 

 movements in the crust ; faults or joints to which Daubree had given the 

 designation " lithoclases." In no case were these fractures found to be 

 contraction fissures, which Werner and many subsequent writers assumed 

 to be the tpyical vein fissure, disregarding the consideration that con- 

 traction fissures could not traverse two distinct bodies of rock. To the 

 joint-like fissures that are confined to a single bed, Whitney had already 

 given the name " gash " veins. 



In the Comstock Lode report, Becker had discussed mathematically 

 the mechanics of faulting as applied to vein fissures, and had shown that 

 an important characteristic of faulting on a fissure in solid rock is the 

 tendency of the movement to separate the rock into sheets by subordinate 

 fissures parallel to the main one. From practical observation Emmons 

 had similarly concluded that the faulting movement which produced 

 vein fissures was often distributed on a number of parallel fissures, thus 

 producing a sheeting of the country rock. Where these fissures were 

 sufficiently close together, so that the intermediate sheets of country 

 rock were very thin and had been partially replaced by vein material, a 

 banding would result which might be mistaken for that of the t}'pical 

 vein of incrustation. W 7 here they were farther apart and of approxi- 

 mately equal strength, the mineral filling, instead of being confined to a 

 single fissure, might be distributed on several, thus rendering frequent 

 cross-cutting advisable in their exploitation. 



The idea that later formed ore deposits are necessarily the filling of 

 considerable cavities or open spaces in the enclosing rocks has been con- 



