J. P. Iddings — Origin of Quartz in Basalt. 219 



solidify the magma more or less completely for the time being, 

 but which, as the quartz grains themselves show, was an un- 

 stable solidification, which subsequently yielded to the poten- 

 tial liquidity of the magma, resulting in the partial resorption 

 of the quartz crystals before the final consolidation of the rock 

 to its present form. 



Reyer, (1. c. p. 166), suggests that pressure and different 

 degrees of saturation with absorbed water may lead to meta- 

 meric processes. And also states that the development of 

 quartz in rock magmas requires a considerable saturation of 

 the magma with water. 



Prof. Lagorio (1. c.) refers the concurrence of quartz and oli- 

 vine in the same rock to the super saturation of the magma 

 with silica and magnesia, but this idea of itself is not sufficient 

 to account for the occurrence of quartz in magmas with the 

 normal basaltic composition, where it generally does not occur, 

 as it is not sufficient to explain the occurrence of fayalite in 

 rhyolitic obsidian, having 75. per cent of silica and less than 

 2 per cent of iron oxide. 



Confirmatory observations. — If the foregoing explanation 

 which refers the production of the quartz in these basalts to 

 physical conditions apart from chemical ones is correct, we 

 should expect to find such anomalous associations of minerals 

 in other varieties of volcanic rocks, and should not expect to 

 find a necessary correspondence in the chemical composition of 

 all basalts which carry porphyritic quartzes. Nor should we 

 expect to find the quartz-bearing varieties, which are excep- 

 tional, necessarily holding a definite relation in point of age to 

 the other volcanic rocks with which they are associated. These 

 expectations, I think, are realized by the following observa- 

 tions. 



Porphyritic quartz in other volcanic rocks. — The first point 

 is beautifully illustrated in the suite of rocks in the collection 

 from New Mexico, for it shows that similar quartz grains occur 

 in almost all of the varieties of volcanic rocks from this region, 

 and that their occurrence is not uniform throughout the series. 



Thus in most of the rhyolites rounded grains of quartz 

 are very abundant, but in some specimens they are absent 

 (obsidian and lithoidite). In the single specimen of mica-ande- 

 site they are wanting. In the hornblende-mica-andesites, some 

 specimens show a considerable number of quartzes; one, a 

 few, and others, none. In the hornblencle-pyroxene-andesites, 

 one specimen shows many quartz grains; others, considerable; 

 and some, none. Of the Hye specimens of pyroxene-andesite, 

 one shows a few grains of quartz ; the rest, none. And of the 

 twelve specimens of basalt, seven show much quartz ; the 

 others, none. 



