88 



GEOLOGIC GUIDEBOOK ALONG HIGHWAY 49 



[Bull. 141 



One point on which geologists do not agree concerns the sources of 

 the water and silica. One school of thought believes that all the water and 

 silica came from deep magmas, bringing with them the gold and other 

 metals. A second school believes that most of the water by which the vein 

 materials were carried in solution percolated into the fissures from the 

 rocks of the surrounding country ; and that the silica was dissolved from 

 rocks adjacent to the fissure through the agency of carbonic acid gas that 

 was contained in the solutions. Both theories are well reasoned and are 

 based on close observations, and either is credible when read by itself. 



Another point on which agreement has not been reached is the way 

 in which the vein fissures have been opened wide enough to contain veins 

 of the widths that are found in them. Open unfilled fissures do not 

 and cannot exist in deep rocks. It is thought by some that the walls of 

 the vein-fissures have been forced apart by a pressure and expansive 

 force inherent in the vein-forming material itself, the force being due 

 largely to the expansive force of compressed gases. By others it is thought 

 that faulting, which is the slipping of one wall of a fissure over the other, 

 caused the separation of the walls, each wall riding on the high spots of 

 the other with a wedge-like effect ; and that the open spaces were filled 





*. .;:*. 



FIG. 2. Mother I/ode quartz vein outcropping in a long vertical reef running 

 from left to right across the hackground of the picture. Such reefs are to be seen on 

 many parts of the Mother Lode. The above outcrop is close to Highway 49 near Carson 

 Hill between Angels Camp and Melones. 



with quartz before they could be closed by a later movement. Movements 

 of this kind, followed by cementation by quartz, if repeated over and 

 over, could account for the widths in which the veins are found. Whatever 

 is the true explanation, a stupendous force was required to push the 

 walls apart. 



Perhaps these problems will never be fully cleared up because the 

 true explanations depend so much on facts and conditions that are only 

 partly known, and on processes that are arrived at only by deduction. 

 The case is somewhat similar to that known in criminal law as one of 

 circumstantial evidence, a verdict being convincing as far as sufficient, 

 reliable evidence is available. But for general information this much can 

 be said with confidence, and this is my answer to the opening question : 

 The veins were deposited in deep fissures, far below what was then the 

 surface of the earth, from hot, ascending, water solutions containing 

 silica and other mineral substances; that these substances remained in 

 the solution while it was hot and under high pressure ; and that the 

 quartz and gold was deposited in the higher parts of the fissures because 

 of a lowering in both temperature and pressure. 



FIG. 3. ''White Rock" a quartz vein outcropping in the vicinity of 

 Indian Gulch, Mariposa County. 



