320 DYNAMICAL GEOLOGY. 



Again, dikes of trap and other igneous rocks have undergone metachemic alteration 

 through interior heated vapors which ascended with .the rock, making the rock hydrous 

 and producing other clianges, and by the same means its vesicles have often been filled 

 -with, secondary minerals made out of the materials of the rock. 



Mineral springs are often referred to as a source of outside ingredients. 

 But, for regional work, such springs should have a wide distribution ; other- 

 wise the effects would be local. One mineral spring, rich in salts of soda 

 and magnesia, has already been mentioned as the great one of the world — 

 the ocean. Nearly all the sedimentary rocks were made in it, or have at 

 some time been submerged in it. Moreover, there is evidence that salt 

 water has an extensive subterranean distribution, in the fact that a large 

 part of the borings for gas and oil, which have been made in recent years, 

 have encountered salt water below depths of 1000 or 2000 feet — depths too 

 great to be made fresh by subterranean drainage. Further, formations of sev- 

 eral geological periods contain great beds of rock salt that were beyond doubt 

 of oceanic origin. 



Associated with the salt, or in the same series of rocks, there are some- 

 times deposits of magnesian salts of like oceanic origin ; and, more spar- 

 ingly, of potash salts; and also of boron salts, for the magnesium borate, 

 boracite, occurs in salt mines, and other boron salts exist in hot springs, 

 sometimes in volcanic emanations, — facts that point to a marine source. 

 Moreover, traces of borates have been detected in the ocean's waters. 

 The beds of salt and the briny layers are interstratified, sometimes in 

 many alternations, with shales, sandstones, and limestones ; and it is 

 natural, therefore, that the soda and magnesia should be forced to take part 

 in any chemical changes the associated formations might undergo. Meta- 

 morphic work may have derived much soda from this source for making 

 soda-lime feldspars, as oligodase and lahradorite ; supplies of magnesia for 

 forming hornblende and black mica ; smaller supplies of potash for ortho- 

 clase-making ; _and still smaller of boron, yet enough to account for the wide 

 distribution of tourmaline, whose constituents, apart from the boron, differ 

 little from those of garnet, — a mineral that is common in mica and chlorite 

 schists, crystalline limestone, quartzyte, and other rocks. 



The vyide distribution of alkaline waters over the Great Basin (page 119) suggests 

 another available source of materials, and especially of soda and magnesia. But such 

 regions are a consequence of the absence of drainage, and could exist only in great lands 

 like continents ; they, therefore, belong only to the latter end of geological time. 



The following are examples of metachemic work in crystalline rocks. Massive talc, 

 called rensselaerite, at Fowler, Dekalb, and other places in northern New York, made from 

 pyroxene, whose cleavage it has ; a pinite, called gieseckite, at Diana, N. Y., and in Green- 

 land, made from crystals of nephelite, the form remaining ; pinite also from scapolite, 

 at Franklin, jST. J. (algerite) and Arendal, Norway ; chlorite in many localities, from 

 garnet (crystals being sometimes chlorite outside only, and sometimes throughout), 

 pyroxene, hornblende, etc. ; mica and epidote from scapolite, at Arendal, Norway ; 

 and feldspar from scapolite, at Bamle, Norway ; epidote from biotite-mica ; diaspore, 

 margarite, and other species, from corundum. In a large granitic vein at Branchvilie, 



