318 DYNAMICAL GEOLOGY. 



persthene also occurs altered to hornblende ; cyanite to andalusite ; labradorite and anor- 

 thite to saussurite. Quartz changes to tridyinite, or to biaxial silica, when it is heated 

 to or above 2200° F., which accounts for the occurrence of tridyinite in volcanic rocks ; and 

 tridymite, or biaxial silica, becomes uniaxial optically at 200° F. ; but further than this 

 it does not change on heating. 



Pyroxene, a common volcanic mineral, has long been known as a furnace product, 

 and since 1823 as the result of the fusion of its constituents ; but Deville, in 1858, obtained 

 crystals by simply heating to a bright red heat a piece of ferruginous Fontainebleau sand- 

 stone with chloride of magnesium ; the crystals of pyroxene cemented the quartz grains 

 of the sandstone. Hornblende has never been produced experimentally from fusion ; but 

 in 1890 the Russian chemist, Krustchoff, heated together its constituents for three months, 

 at a temperature of only 900° to 1000° F., and obtained hornblende in crystals ; and along 

 with them were crystals of quartz and of a light-colored pyroxene (diopside). The facts 

 show that the change of pyroxene to hornblende requires only a heating of the rock con- 

 taining it to 1000° F. It indicates also that if a pyroxene-bearing rock, on cooling from 

 fusion, rests long at this temperature, it would probably become throughout a hornblende 

 rock, and appear as if so primarily. 



Paramorphic metamorphism should be of common occurrence ; for paramorphs are 

 essentially identical except in crystallizations (pages 62, 67, 69). . 



4. Metachemic Metamorphism. 



Through the chemical work of metamorphism have been made nearly all 

 the common siliceous minerals among rock constituents, even many kinds 

 that are also of igneous origin : as the feldspars, micas, quartz, minerals of 

 the hornblende and pyroxene group, the chlorites, epidote and the related 

 species, scapolites, garnets, tourmaline, chrysolite, and many others. The 

 older metamorphic and igneous rocks have been the chief sources of the 

 materials. Even in formations not older than the Cretaceous, as described 

 in Becker's account of the rocks of California, the results of metamorphism 

 include orthoclase, albite, oligoclase, labradorite, muscovite, biotite, horn- 

 blende, pyroxene, glaucophane, epidote, zoisite, garnet, chlorite, serpentine, 

 talc, and other species. 



In these metachemic changes, without aid from outside ingredients, feld- 

 spar may be altered to hydromica, or mica (muscovite), under metamorphic 

 action (Van Hise). Each of these minerals contains silica, alumina, and 

 potash, but the mica, a third less of silica ; hence a feldspathic or granitic 

 sandstone may be made micaceous, and a feldspathic shale may be converted 

 into a hydromica or mica schist. Hydromica schist is a common rock in the 

 regions of crystalline rocks of eastern North America, and in other such 

 regions over the globe ; and feldspathic sediments, derived from the abundant 

 feldspar of these rocks, are their only source. Eor mica scales float easily in 

 transporting waters and become scattered among other materials instead of 

 being gathered together into beds. A felsyte may change to pinite, as near 

 Boston (Crosby), which mineral is essentially a massive mica. 



But if the shale is an argillaceous rock without potash (no undecom- 

 posed feldspar being present) it is very likely to contain more or less iron, 

 magnesia, and lime ; and then it has the elements required for making a 



