8l6 THE JOURNAL OF GEOLOGY. 



affect the orientation of the cement. Where the rock is coarser 

 grained, as is the case in some of the basic volcanics, the charac- 

 ter of the cement can be directly tested and the material proved 

 to be quartz. 



While in some cases this structure is undoubtedly of primary 

 character, as Professor Iddings considers it to be in many 

 porphyrites, in a large class of rocks its secondary origin seems 

 equally plain. Dr. Irving, who very early described this struc- 

 ture in the acid lava flows of the Keweenawan series, thus speaks 

 of its origin. 1 " Whether this secondary quartz may ever be 

 rather a result of devitrification than a truly secondary or alter- 

 ation-product I have no means of deciding, though it is certainly 

 the latter often, and I should suppose always. It surely can 

 have no connection with the original solidification of the rock." 

 Observations made on the South Mountain rocks likewise point 

 to a secondary origin for these quartz areas. As the origin of 

 the structure is of importance in its bearing on the question of 

 the primary or secondary character of the crystalline ground- 

 mass, these observations will be briefly mentioned. In a speci- 

 men of basic lava from the railroad tunnel near Monterey the 

 outline of lath-shaped feldspars forming an ophitic structure, 

 which is undoubtedly original, is completely preserved. None 

 of the original constituents of the rock remain, however, unless 

 some of the titaniferous iron oxide is original. The rock con- 

 sists entirely of quartz, epidote, magnetite (or ilmenite), and 

 leucoxene. The quartz acts as a cement for the other minerals, 

 forming irregular interlocking areas which are quite similar to 

 the micropoikilitic areas of the acid rocks and which produce in 

 polarized light the familiar patchy effects. Fine cracks traversing 

 the rock, and parting the ferro-magnesian phenocrysts (now repre- 

 sented by epidote) are plainly prior to the quartz areas in which 

 they become invisible. There can be no question as to the 

 secondary character of the micropoikilitic structure in this case. 



In the acid rocks the quartz areas are frequently more or less 

 oval and outlined by a microfluidal arrangement of globulites, 



1 Opus cit., p. ioo. 



