GRAYWACKE. 881 



t.o similar orientation, they usually do not become regularly parallel. The 

 slight tendency toward a parallel arrangement usually accords with bed- 

 ding, but in some cases it may not do so. The banded character of the 

 original rock is ordinarily preserved, even if all of the decomposable detrital 

 minerals are altered into new ones. Each mineral may change into any of 

 the products which form in the belt of cementation, as listed. (See pp. 

 621-627.) To illustrate, the feldspars may alter into quartz, chlorite, the 

 zeolites, and the epidotes; the pyroxenes and amphi boles may alter into 

 quartz, serpentine, calcite, dolomite, magnesite, magnetite, and the epidotes; 

 the micas may alter into serpentine, chlorite, and quartz. These secondary 

 minerals vary from a subordinate to a dominant amount, and in some 

 instances but little if any of the original feldspars, pyroxenes, amphiboles, 

 and micas remain. But in general it is difficult to discriminate the meta- 

 somatic changes which take place in grits and graywackes from the partial 

 alterations of the minerals before the material was deposited as a ferro- 

 magnesian sand. Where a ferromagnesian mineral of a certain kind is very 

 abundant the alteration products from such a mineral are correspondingly 

 plentiful. 



Secondary minerals which form from several important primary min- 

 erals are likely to be especially abundant; as, for instance, serpentine. 

 This mineral very often develops on a considerable scale in the gray wacke.s. 

 Indeed, it sometimes becomes so prominent that the metamorphism of gray- 

 wackes has been called serpentinization. The secondary serpentine acts in 

 a manner similar to that of silica. It may occupy the crevices and cracks, 

 the spaces between the constituent minerals, and, finally, the place once 

 taken by other minerals. Exactly as in the case of silica, material for the 

 serpentine may be furnished in part or in whole by the minerals present, or 

 the material of the serpentine may come from an extraneous source, espe- 

 cially from the belt of weathering. Widespread formations may be exten- 

 sively serpentinized, so as to give for considerable areas almost solid masses 

 of serpentine. The serpentinized graywackes are especially well illustrated 

 in the Coast Range of California. " (Fig. 23.) 



Excellent illustrations of the graywackes may be found in the Huronian 

 of the Lake Superior region, and perhaps the best illustrations of all are 



« Becker, G. F., Geology of the quicksilver deposits of the Pacific Slope: Mon. U. S. Geol. Survey, 

 vol. 13, 1888, pp. 120-128. 



MON XLVII — 04 56 



