SUMMAEY. 467 



where in this region is there any evidence of the suhstitution of ore for 



rock. 



New Aimaden district. — Thc first cliscoverecl and the most productive of the 

 quicksilver mines of North America is the New Aimaden, and in the same 

 district the Guadalupe, Enriquita, and other mines have yielded quicksilver. 

 The district is well watered and wooded and is more attractive than any 

 other of the quicksilver camps. 



Upon highly metamorphosed rocks he Miocene sandstones, which were 

 sharply folded at the Post-Miocene upheaval. They are not conformahle 

 with the lower series and contain pebbles from these older beds. In the 

 older rocks near New Aimaden Mr. Gabb found Aucdla, proving the pres- 

 ence of the Knoxville series. 



In this district is the only mass of rhyolite thus far found in the Coast 

 Ranges. It forms a dike nearly parallel to the line connecting the New 

 Aimaden and the Guadalupe. It is almost continuous, and I have followed 

 it for a distance of several miles. It is certainly Post-Miocene and prob- 

 ably Post-Phocene. 



The New Aimaden is a very extensive mine, said to contain as much 

 as 40 miles of galleries. Much of this length is open, and admirable op- 

 portunities are atforded for study of the ore and structure. The ore is 

 cinnabar, with occasional traces of native quicksilver, accompanied by 

 pyrite and marcasite, with rare crystals of chalcopyrite. The gangue is 

 quartz, calcite, dolomite, and magnesite. These materials are deposited in 

 shattered masses of pseudodiabase, pseudodiorite, serpentine, and sand- 

 stone. There is no deposition by substitution and impregnations are very 

 subordinate. Considered in detail, the ore bodies are stockworks ; but 

 they are arranged along definite fissures and the deposits as a whole have 

 a vein-like character and answer to the " chambered veins " defined in a 

 subsequent paragraph. The workings have developed two main fissures. 

 One of these dips from the surface at a high angle and in a nearly straight 

 line. The other strikes in nearly the same direction as the first, dips 

 steeply from the surface, then flattens and approaclies the first fissure rap- 

 idly, ao-ain becomes very steep, and in the lowest workings almost coincides 

 with the first. In vertical cross-section the two fissures foi-m a figure 



