Page:The Journal of geology (IA journalofgeology11893univers).pdf/400

affect the orientation of the cement. Where the rock is coarser grained, as is the case in some of the basic volcanics, the character 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 structure 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 alteration-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 groundmass, these observations will be briefly mentioned. In a specimen 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 consists 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 represented 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,