92 QUICKSILVEll DEPOSITS OF THE rACIFIO SLOPE. 



No. 212a, Sulphur Bank, is a dull-green sandstone intersected by 

 quartz veins. Under the microscope it is seen to be a partialh^ decomposed 

 sandstone, containing numerous fragments of quartz, orthoclase, and pla- 

 gioclase, the interstices being in part filled with serpentine. The slide is 

 chiefly remarkable for the presence of zoisite in considerable quantities, 

 growing into the quartz grains, Tlie zoisite shows the characteristic extinc- 

 tions, interference colors, refraction, and cross-section. 



No. 15, Sulphur Bank, an ordinar}^ gray, indurated sandstone, shows 

 plienomena similar to those described in No. 8G, New Idria. It also con- 

 tains a great deal of zoisite, both in the granular and the prismatic form. 

 The zoisite in this slide was tested with nitric acid to make sure that apatite 

 had not been mistaken for it. 



An especially significant rock is No. 13, New Almaden, which is 

 ])ot]i macroscopically and microscopically unquestionably an altered sand- 

 stone. In the slide, however, it is seen that the progress of the metaso- 

 matic recrystallization has been somewhat irregular and that tliere are 

 fields in the slide which could not be distinguished fi'om an ordinary eruptive 

 diabase. As the slide is moved, however, the structure and mineral compo- 

 sition change gradually and by insensible degrees until the eruptive habitus 

 is lost and the clastic character is clearly revealed. There is no suggestion 

 of included fragments or dikes of eruptive rock in the specimen or slide. 



No. 13, Sulphur Bank, a slightly altered sandstone from the meta- 

 morphic series, was selected for chemical analysis. Under the microscope 

 the rock appears to be arcose, the grains being quartz similar to those of the 

 granites, plagioclase, and unstriated feldspars, with the optical properties of 

 orthoclase. The grains are cemented by newly formed aggregates which 

 seem to consist in great part of triclinic feldspar. From inspection one 

 would expect to find in this rock about equal quantities of soda and pot- 

 ash. The following analysis, however, shows a ver}' diff"erent and unex- 

 pected relation : 



Lossiit 100°, H-O 0.276 



Lossabovo 10U-, H-O 2.11:? 



Silica, SiO" 08. iiOO 



Phosphoric acid, P=0» 0.163 



Titanic aciil, TiC 0.600 



