MODIFICATIONS OF QUARTZ. 219 



the greatest consequence. Some of the more common minerals in which 

 silication occurs on an extensive scale are calcite, dolomite, ankerite, and 

 siderite, thus producing wollastonite, diopside, tremolite, sahlite, actinolite, 

 and griinerite. The silica may unite with the bases of various carbonates 

 producing various complex silicates, such as chondrodite, augite, horn- 

 blende, garnet, etc. At the same time the material of previous silicates 

 may be absorbed. The heat and volume reactions in many of these 

 changes may be found under the carbonates mentioned. 



In this process of silication of carbonates it is not often possible to 

 identify the remnants of the quartz individuals which furnished the silica 

 for the reactions. But apparently the quartz particles which furnished the 

 silica for the process of silication may be identified in some instances. 

 This is best seen for such fibrous minerals as serpentine, talc, and actinolite, 

 the needles or fibers of which appear to grow into the quartz, in some 

 instances deeply. In such cases it seems clear that the silica of the quartz 

 furnished at least a part of the silica for the silicate, the bases being 

 furnished by the solutions. 



One of the best instances of the extensive union of quartz with bases, 

 producing serpentine pseudomorphous after quartz, is that described by 

 Becker." He describes the exteriors of original clastic grains of quartz to 

 be "entirely occupied by felted fibers of serpentine, and long, slender 

 microlites pierce the quartz grain toward its center, like pins in a cushion." 6 

 This is but one illustration of a very widespread replacement of quartz 

 by serpentine in the Coast Ranges. The growth of actinolite into quartz 

 is illustrated in the Tyler slate of the Penokee district of Wisconsin." 



In instances where the quartz furnishes the silica for the penetrating 

 silicates the migration of the silica is microscopical, and it might be sup- 

 posed that the reactions occur without the solution of the silica of the 

 quartz; but it seems probable, even in such cases as these, that there is 

 solution of the silica before combination with the bases. In such reactions 

 it is presumed that the bases which unite with the silica were before united 

 with some other acid, and it is only when the previous combination is known 

 that the heat and volume relations of the reactions can be ascertained. 



« Becker, G. F., Geology of the quicksilver deposits of the Pacific slope: Mon. Ui S. Geol. 

 Survey, vol. 13, 1888, pp. 120-127. 



b Becker, cit., p. 124. 



« Irving, R. D., and Van Hise, C. R., The Penokee iron-bearing series of Michigan and Wisconsin: 

 Mon. U. S. Geol. Survey, vol. 19, 1892, pp. 210-215. 



