316 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



ing- a large amount of interstitial space, ore bodies were formed, but wliei'e 

 the rock yielded to stress as a plastic mass no room was left for ore. 



Aitas. — The ore in tiie New Almaden mine seems never to occiu- except 

 close to evidences of foulting. This evidence consists in the presence of 

 layers of attrition products, so-called clays, full of slickensides and of frag- 

 ments of rocks more or less rounded bv attrition. These layei's of clay 

 usually occur on the hanging side of deposits and are known to the miners 

 as ultas, the Spanish term for hanging- walls. The clavs are impermeable to 

 solutions and the ore usually forms on their lower side, as if the cinnabar 

 had ascended and been arrested by the alias. That the solutions really 

 took this course is clearly shown by the phenomena of otlier quicksilver 

 districts, as well as by the relations observed in the New Almaden mine 

 The miners very properly follow seams of alta in their search for ore. 

 Sometimes, however, a second mass of ore exists on the hanging side of 

 the clay and is again limited by a second layer of alta, as I have myself 

 observed. Such occuri-ences are to be expected in a country so irreg'ularly 

 disturbed as this. The alta is not a definite substance, though it is usually 

 a dark or black mass, readily distinguishable even in hand specimens from 

 the countr\" rock. It is simply triturated country rock and varies in com- 

 position with the material from which it has been produced. Its black 

 color is in part due to the presence of manganese. These layers of clav 

 correspond exactly to those which were met with in the upper portion of 

 the Comstock lode and against which many bonanzas were found to rest. 

 The evidence of movement in the New Almaden mine is not confined to 

 clays. Where the opposing walls were so nearly parallel that no consider- 

 able quantity of trituration took place, polishing occurred, and some of the 

 slickensides met with are as brilliantly polished as if the work had been 

 done by a lapidary. 



Form of the New Almaden ore bodies. Wllilc the evideUCC of the CxistenCC of a fis- 



sure system is, if possible, more abundant in the New Almaden mine than 

 in most other quicksilver deposits of the Pacific slope, the deposits them- 

 selves are of various types. The commonest is the reticulated mass, or 

 stockwork, consisting of irregular bodies of broken rock into which solutions 

 of cinnabar and gangue minerals have filtered, cementing the fragments to- 



