310 DYNAMICAL GEOLOGY. 



and beyond the Massachusetts line — Cambrian fossils in the sandstone or 

 quartzyte, and Cambrian and Silurian in the crystalline limestone or marble 

 belt next west ; and a few miles farther west they occur going southward in 

 limestones and schists for 150 miles to Poughkeepsie in Dutchess Co., N. Y., 

 and beyond. In the more crystalline parts of the same region to the east- 

 ward in Massachusetts, the quartzyte graduates into gneiss and alternates 

 with mica schists, and the slates change to staurolitic mica schists and 

 gneiss. The fossils in the Taconic region were found by A. Wing, Walcott, 

 Dwight, Dale, Wolff, and others. 



Again, near Bernardston, Mass., and the region northward along the Connecticut 

 Valley, Crinoids and Brachiopods occur in a crystalline limestone of Devonian age, asso- 

 ciated with hydromica schist, gneiss, granite, dioryte, hornblende schist, quartzyte, all of 

 one Devonian series, and of synchronous metamorphism. (E. Hitchcock, B. K. Emerson.) 



In the Alps, at the St. Gothard tunnel, crinoidal remains occur in calcareous mica 

 schist (Muller). In the Apuan Alps, Orthocerata exist in limestone between beds of 

 gneiss and mica schLst (Meneghini). At Brevig, Norway, a Silurian limestone contains 

 garnets, scapolite, and fossils, and, according to Reusch, mica schist containing Halysites, 

 Favosites, Gydthophyllum, MurcMsonia, Calymene, Dalmanites. Schists, in Brittany, 

 afford andalusite crystals and species of Orthis, Spirifer, and Calymene, in one and the 

 same specimen (Boblaye). At Rothau, in the Vosges, in a hornblende rock, corals occur 

 replaced, as stated by Daubree, without losing their form, by crystals of hornblende, 

 garnet, and axinite, and among the corals the species Calamopora spongites is quite 

 distinct. 



The rocks that have become changed into metamorjDhic rocks are for the 

 most part the fragmental rocks, as sandstones, shales, conglomerates, with 

 the limestones. These, according to their various constitution, have been 

 changed to gneiss, granite, mica schist, and the several other kinds of schist ; 

 and the limestones to crystalline limestones ; and this change has been the 

 chief method of origin of the schists. In addition, the many crystalline 

 rocks, both the metamorphic and igneous, have undergone, to some extent, 

 related changes. 



Under metamorphism might be included the chemical changes in rocks 

 and minerals that take place at the ordinary temperature. But these run 

 down into the common results of decay, and are more conveniently kept 

 separate. They have been described on page 118 and beyond. 



Causes of Metamorphism. 



1. Not generally due to infiltrating waters. — The metamorphic changes 

 which rocks have undergone is no evidence of their instability under existing 

 conditions. It has been already shown that the sandstone, shales, and 

 other fragmental rocks are seldom so porous at depths below as to admit the 

 passage of infiltrating waters. It is true also of the crystalline rocks, 

 granite, gneiss, syenyte, and the various igneous rocks, that they are com- 

 monly too close in texture to admit the passage of underground waters. The 

 moisture they hold is stable, and the rocks are stable against changes from 



