igoS.] THE PHYSICS OF THE EARTH. 173 



mountains, volcanoes sometimes break out. But it is obvious that 

 earthquakes are the more general, volcanoes the more special phe- 

 nomena; and that both are connected with mountain formation, and 

 depend on the sea for their continued activity. 



§ 9. On the Structure of Granite as a Typical Crystalline Rock 

 of the Earth's Crust. — Granite has a thoroughly crystalline structure, 

 and is an admixture of feldspar, mica, and quartz. The mica is in 

 the form of minute shingles, or snowflakes, embedded in the non- 

 crystalline matrix of quartz, which encloses the other elements. The 

 feldspar is chiefly orthoclase. The two chief ingredients, quartz and 

 feldspar, form a granular aggregate made up of grains of fairly 

 equal size, varying all the way from several inches in diameter to a 

 structure so fine as to be inseparable to the naked eye. 



"Many granites contain irregularly shaped cavities (miarolitic structure), 

 in which the component minerals have had room to crystallize in their 

 proper forms, and where beautifully terminated crystals of quartz and 

 felspar may be observed. It is in these places also that the accessory 

 minerals (beryl, topaz, tourmaline, garnet, orthite, zircon and many others) 

 are found in their best forms. Not improbably these cavities were some- 

 what analogous to the steam holes of amygdaloids, but were filled with water 

 or vapour of water at high temperature and under great pressure, so that 

 the constituents could crystallise under the most favorable conditions. 

 Among the component minerals of granite, the quartz presents a special 

 interest under the microscope. It is often found to be full of cavities con- 

 taining liquid, sometimes in such numbers as to amount to a thousand 

 millions in a cubic inch and to give a milky turbid aspect to the mineral. 

 The liquid in these cavities appears usually to be water, either pure or con- 

 taining saline solutions, sometimes liquid carbon-dioxide" (p. 143). (Sir A. 

 Geikie, "Geology," p. 204.) 



The cavities in crystalline rock such as granite may contain 

 either gas or liquid matter, and sometimes both. Professor Tilden's 

 researches have shown that the included gases (hydrogen, carbon 

 dioxide, carbon monoxide, marsh gas, nitrogen, and water vapor) 

 may exceed many times the volume of the rock itself. The cavities 

 have all manner of forms, branching, oblong, curved, oval, spherical 

 and negative crystalline shapes, and are often so numerous as to give 

 a turbid aspect to the mineral. The intersecting planes of the crys- 

 talline granite frequently present real fissures more or less filled with 

 liquid. Obviously capillary forces may here attain great importance, 

 and fluid entering the rock would be absorbed into these spaces 



PROG. AMER. PHIL. SOC. XLVII. 189 L, PRINTED SEPTEMBER 21, I908. 



