STEUCTURAL FEATURES OF IGNEOUS BOOKS 57 



haps microcrystalline or even glassy, at the immediate contact. 

 These two phenomena may afford the only means of determining 

 whether a rock mass occurring in the form of a sheet between 

 sedimentary beds, is an intrusive or a contemporaneous lava 

 JS.OW; whether it was injected between two previously existing 

 beds; or whether, as a lava flow, it was poured out over the 

 lower, first formed, after which the second was laid down upon 

 its surface. If formed as an intrusive sheet, one may expect to 

 find the rock more dense along both contacts, in addition to 

 which there may be more or less contact metamorphism of the 

 sedimentary beds from the action of the hot intruded material. 

 If poured out as a lava, on the other hand, contact metamorphism 

 and the dense, fine-grained portions will be limited to the lower 

 contacts, while, provided there had been no great amount of 

 erosion between the time of the pouring out of the molten mass 

 as a surface flow and the deposition of the newer sediments, the 

 tipper portions will be less dense, perhaps even vesicular, sco- 

 riaeeous, and glassy, while the sediments themselves, having 

 been laid down on cold consolidated material, remain wholly 

 unchanged. Such means of discrimination have been of the 

 greatest value in ascertaining the relative ages of portions 

 of the Triassic sandstones and associated traps in the eastern 

 United States. 



The lava flows, cooling so much more rapidly than the plu- 

 tonic rocks, owing to their exposed position and relief from 

 pressure, often show but incipient forms of crystallization, or 

 are quite glass-like, as is the case with the obsidians of the 

 Yellowstone Park and elsewhere. Chemically these last are prac- 

 tically identical with granite, but they have cooled too quickly for 

 the forces of crystallization to act. Owing, further, to the ex- 

 pansive force of the included vapor of water, — a constituent 

 of all lavas, — these surface flows are at times so filled with 

 cavities as to be quite pumiceous. The pumice purchased at the 

 drug-stores is but the froth from a lava which, had it cooled 

 slowly and under greater pressure, might have yielded a granite. 



A common feature of the effusive or volcanic rocks is a flow 

 structure, sometimes visible only with the microscope, which 

 is due to a flowing movement of the magma while undergoing 

 consolidation. (See Fig. 2, PI. 2.) The characteristic structure 

 of effusive rocks is porphyritic, instead of granular, and repre- 

 sents two distinct phases of cooling and crystallization: (1) an 



