518 EEPOET OF NATIONAL MUSEUM, 1890. 



(5) Quartz porphyry, Fairfield, Adams County, Pennsylvania. 

 Transparency No. 39078. Section No. 26377. 

 This rock has essentially the same composition as granite, but is of 

 a markedly different structure owing to the different conditions under 

 which it cooled from a molten mass. The large crystals just below 

 the center aud to the left of the top are of quartz, the one on the ex- 

 treme right feldspar, while the main body of the rock is made up of an 

 intimate mixture of these two minerals in the form known as felsitic. 

 This porphyritic structure is characteristic of a large class of what are 

 known as effusive rocks, and represents two distinct stages of crystal- 

 lization. The large porphyritic crystals were formed during or prior 

 to the period of eruption, the intratellurial period, while the very fine 

 grained grouudrnass is due to a more rapid crystallization after the 

 flow had ceased. (See also Fig. 3, PI. cxx.) 



(G) Ehyolite, High Eock Caiion, Nevada. Transparency No. 39079. 

 Section No. 35441. 

 This rock, like the last, shows porphyritic crystals of quartz in a 

 felsitic groundmass, the quartz being brilliant blue in the transparency. 

 The felsitic groundmass, it will be observed, has a spherulitic structure 

 and also a flow of fluxion structure, giving rise to the nearly parallel 

 banding extending from top to bottom aud which is due to the onward 

 flowing of the molten lava while cooling and crystallizing. 



(7) Hornblende Andesite, near Mono Lake, California. Trans- 



parency No. 39080. Section No. 35491. 

 This rock is composed essentially of the minerals hornblende and 

 plagioclase, the former showing in opaque red and yellow crystals with 

 a dark border, while the plagioclase appears colorless or faintly clouded 

 in large irregular and prismatic forms, often showing a banded or zonal 

 structure somewhat resembling the lines of growth upon the trunk of 

 a tree. The extremely fine grained brownish portion is composed of 

 minute imperfectly formed crystals of both hornblende and feldspar in 

 the form called microlites ; hence such a groundmass is called micro- 

 litic. 



(8) Basalt, Bridgeport, California. Transparency No. 39081. Sec- 

 tion No. 256G3. 



This rock has essentially the same mineral composition as the diabase 

 already decribed, with the addition of olivine. Geologically it is differ- 

 ent in having been a surface lava flow and of more receut origin. Ob- 

 serve that the rock is porphyritic and that the groundmass is composed 

 of innumerable small lath-shaped crystals of plagioclase with a small 

 quantity of glassy matter in the interstices. This, as in the quartz por- 

 phyries and liparites, denotes two distinct phases of crystallization. 

 Structures of this kind, produced by porphyritic crystals imbedded in 

 a groundmass in part crystalline and in part glassy, are technically 

 known as hypocrystalline porphyritic. 



