214 J. P. Iddings — Origin of Quartz in Basalt. 



portions of the acid lava are composed of alkali-feldspar, 

 quartz, tridymite and fayalite, an iron olivine. The latter 

 mineral is in very small amount compared with the free silica, 

 quartz and tridymite. 



Influence of absorbed water. — In a paper on this occurrence 

 (this Journal, xxxiii, Jan., 1887) I have discussed the matter 

 at length, and after calling attention to the experiments of M. 

 Daubree on the action of superheated steam on acid glass, and 

 to the experiments of "others who have undertaken to produce 

 these minerals artificially, I arrived at the conclusion, that this 

 anomalous association of primary igneous minerals was most 

 probably brought about by aqueo-igneous action, induced by the 

 influence of water vapor absorbed in the molten glass. The 

 strong mineralizing influence of the water vapor under such 

 conditions becoming apparent both in the structure and com- 

 position of the lithophysse and of the alternating crystalline 

 and glassy layers in the laminated lithoidal portion of the same 

 flow of lava. For in the latter case the thin horizontal lami- 

 nae of the magma in any particular portion of the mass must 

 have been subjected to like rates of cooling and similar pres- 

 sure, yet these laminse solidified alternately holocrystalline and 

 glassy. 



Comparison of exceptional occurrences. — The occurrence of 

 iron olivine in a rhyolite with 75 per cent of silica and less 

 than 2 per cent of iron oxide is as remarkable and as excep- 

 tional as that of quartz in the form of porphyritic secretions 

 in basalt. They are both of the same kind, in that they are 

 the occurrences of extremely acid and basic silicate minerals 

 together in rocks, where we are generally accustomed to see 

 silicate minerals of intermediate or of more closely related 

 composition. 



In the first instance, however, most of the conditions under 

 which the minerals crystallized can be surmised, but in the 

 second case there is much more uncertainty. For the quartz 

 was evidently crystallized in some early period in the history 

 of the rock, when the conditions then existing made its separa- 

 tion from the magma necessary. 



Indications of altered, physical conditions. — From the 

 rounded form of the quartz grains it is probable that the 

 quartzes were being resorbed by the basic magma when the 

 final solidification of the rock took place. 



A similar resorption of porphyritic minerals, which have 

 crystallized in an earlier period of a rock's existence is of com- 

 mon occurrence in nearly all volcanic rocks, as for example, 

 partly resorbed hornblendes and feldspars, and the rounded 

 quartzes of rhyolites and porphyries.' And the idea that these 

 phenomena indicate changes in the physical conditions an(J not 



