G. F. Becker — Metamorphic rocks of California. 353 



altered sandstones. It forms in the aggregates which result 

 from the clastic grains and its microlites sometimes pierce quartz 

 grains from the outside. It is abundant in the granular as well 

 as in the prismatic form. A sandstone from New Almaden has 

 been subjected to somewhat irregular alteration. Although 

 the rock as a whole is both macroscopically and microscopically 

 a fragmental one, there are fields in the slide which have the 

 mineralogical and structural character of an eruptive diabase. 

 These areas do not represent inclusions but pass gradually 

 over into others in which the clastic character is unquestionable. 



It is only necessary to suppose the processes indicated above 

 carried further to obtain a product in which the clastic char- 

 acter of the rocks would cease to be evident. The altered 

 sandstones thus form under the microscope as they do in the 

 field transitions from the clastic series to the holocrystalline 

 rocks. 



The granular metamorphic rocks of the Coast Ranges are sep- 

 arable under the microscope into several groups, but this is not 

 practicable by unaided vision ; indeed there are many cases in 

 which specimens which appear to the naked eye to be not 

 greatly altered sandstones prove under the microscope to be 

 holocrystalline rocks with none of the microstructure of a 

 sandstone. The most important class of the granular rocks is 

 a metamorphic diabase in which the pyroxene sometimes 

 assumes the form of diallage. None of this rock has been 

 found in place which carries olivine, but in one district occur 

 rounded pebbles of an olivinitic gabbro, which is probably of 

 metamorphic origin. This is the only olivinitic rock excepting 

 basalt met with in this region. The metamorphic diabase in a 

 large proportion of cases carries much zoisite. It also fre- 

 quently contains hornblende. Metamorphic diorites are also 

 not infrequent, and in some cases the quantity of hornblende 

 so greatly exceeds that of the other constituents that the rock 

 deserves the name of amphibolite. Glaucophane occurs in 

 both the diabasic and the dioritic rocks. The quantity of 

 zoisite in these rocks is very variable, and in some cases is so 

 great that with feldspar it forms almost the entire mass. The 

 schistose metamorphics, not including phthanites, are all char- 

 acterized by the presence of glaucophane. In every case but 

 one, zoisite is associated with the glaucophane in this group, 

 and either muscovite or biotite is usually present. 



The phthanites or silicified shales form a very distinct group 

 readily distinguishable from the granular metamorphics. They 

 usually are green or brown in color and intersected by in- 

 numerable quartz veins. They contain microscopic organic 

 remains, and imbedded in the quartz veins or projecting from 

 their walls are often numerous zoisite crystals. All of these 



Am. Jour. Sol— Third Series, Vol. XXXI, No. 185.— May, 1886. 

 23 



