HANDBOOK FOR THE DEPARTMENT OF GEOLOGY. 51 9 



The large cluster of blue, scarlet, and yellowish crystals at the left is 

 augite, the scattering greeu, crimson, and brownish ones are olivine, 

 while the abundant smaller lath-shaped forms are of feldspar (plagio- 

 clase.) Observe that the olivines and augites, being among the first 

 minerals to solidify, have formed, as it were, little islands, around which 

 flowed the still molten magma bearing the lath-shaped feldspars like 

 logs in a millstream. 



(9) Gneiss, West Andover, Massachusetts. Transparency No. 



39082. Section No. 26595. 

 This rock has the same composition as granite, but differs in that its 

 constituents are arranged in more or less parallel bands ; that is, it has 

 a foliated or schistose structure. This structure was formerly supposed 

 to be due to the fact that such rocks were metamorphic; that they 

 originated from the crystallization of sediments and were not forced 

 up in a molten state, as was the case with granite. While it is very 

 probable that certain of the gneisses may have been formed in this man- 

 ner, there is little doubt that the foliation in this particular rock is due 

 to dynamic agencies, rather than to original bedding, (See also Plate 

 cxxiv and Fig. 94, p. 547.) 



(10) Mica schist, Washington, District of Columbia. Transparency 

 No. 39083. ' 



This rock differs from the last only in the absence of feldspar, being 

 composed wholly of quartz aud mica. Like the gneiss it is regarded as 

 a metamorphic rock aud possesses a banded structure, which does 

 not show in the transparency, owing to the extremely small part of the 

 rock included. 



(11) Sandstone, Portland, Connecticut. Transparency No. 39084. 

 Section No. 2G077. 



The mineral composition of this stone is quite complex, but the in- 

 gredients are essentially those of granite or the gueisses. The colorless 

 portions are quartz or feldspars, the bright iridescent shreds are white 

 mica, the brownish and opaque, black mica, while the opaque material 

 in the interstices is the ferruginous cement. 



Observe that instead of a mass of interlocking crystals, as in the 

 cases already described, we have here a confused aggregate of crystal- 

 line fragments cemented to form a rock of secondary origin. Such are 

 called classic or fragmental rocks. (See also Fig. 92, p. 536.) 



(12) Quartzite or quartz schist. Berks County, Pennsylvania. 

 Transparency No. 39085. Section No. 26478. 



Quartzites result from the induration of siliceous sandstones. In 

 some cases the granules are elongated and arranged in nearly parallel 

 layers, giving rise to a schistose structure, as here shown. Composi- 

 tion nearly pure quartz. 



