272 GEOLOGY OF THE COMSTOCK LODE. 



sugar-quartz. — TliB iiiicroscope ftu-tlier shows that the bugar-quartz is com- 

 posed of crushed crystals,' and this can also be demonstrated macroscopi- 

 cally. In interstices between fragments of country rock, bunches of quartz 

 crystals are not unconnnon, and these though fractured are sometimes held 

 together by tlie support of the surrounding material. In such cases the same 

 crack can sometimes be observed running through a considerable number of 

 crystals, proving, if necessary, that they have not yielded to an internal 

 stress, but to an external force. Though the whole country is greatly 

 broken up, so that the average size of" the blocks of country rock showing 

 no fissures is not much above the size of a man's fist, it is nowhere reduced 

 to the fineness of sugar-quartz. This need cause no surprise, however, for 

 miners and mill men are well aware that, in spite of its hardness, quartz is 

 very readily crushed, far more readily than volcanic rocks, or even than 

 limestone. The occurrence of sugar-quartz, then, is an evidence of move- 

 ment, and this can have taken place only in one direction, that of the dip 

 of the fissure; for even if it were conceivable that the whole country in this 

 neighborhood might be compressed latterly, the behavior of the quartz in 

 the upper levels would prove the supposition inapplicable. The quartz- 

 sheets which are parallel to the fissure are solid, or, at most, according to 

 Mr. King, show a slaty structure ; while the masses which are not parallel 

 to the fissure are crushed. In some of the bonanzas in other portions of 

 the CoMSTocK, Mr. King noticed a parallelism to the Lode even in the 

 cru.shed masses, and such a phenomenon is also said to have been observed 

 in the great bonanza of this section. 



Period of the fault. — Siucc tlic sccoudary or east fissure was filled with quartz, 

 the faulting action to which the existence of this fissure is due must have 

 preceded the deposition of ore on the Comstock; and since the ore was 

 crushed by a movement in the direction of the dip of the main fissure, fixiilt- 

 mg must also have succeeded the deposition of ore. The fiiulting action 

 studied in Chapter IV. nuist therefore have emln-aced the whole or nearl}- 

 the whole of the period during which the deposition of ore was taking place, 



' The finest portious of t he sugar-quartz Diouuted in bal.saui and examined in pol.arized liglit under 

 the microscope are unmistakably anisotropic, and while portious of crystal-faces are occasionally visible, 

 most of the surfaces are couchoidal fractures. 1 have met with no evidence that any of the solid quartz 

 of the Comstock has resulted from the consnli<latinii of sngar-quartz, either by jire.ssure or any other 

 .'ijiencv. 



