METAMORPHISM. 757 



bonate mostly in the state of aragonite, which differs from calcite in 

 having one thirteenth higher specific gravity, greater hardness, higher 

 specific heat, prismatic crystallization, and no very distinct cleavage. 

 In its metamorphism, as stated by Sorby, the grains become calcite, 

 and the rock then glistens with cleavage surfaces. It is in this change 

 that fossils lose distinctness of forms ; and only a slight degree of it 

 will accomplish the result, since it changes every grain in the fossil. 



7. Crystallization with Changes in the Constituent Minerals of the 

 Rock. — In a large part of metamorphism, chemical forces have had 

 an opportunity to make new combinations (1) out of the ingredients 

 of a sedimentary rock ; or, less commonly, (2) out of these and some 

 introduced ingredients. While the feldspar and quartz of metamor- 

 phic rocks may often have existed as such in the rock before altera- 

 tion, other minerals, including numerous kinds disseminated through 

 the mass, are results of chemical change in the ingredients present. 

 Mica, pyroxene, tremolite, garnet, scapolite, and various other species 

 have been made, in the metamorphism of a limestone, out of its impu- 

 rities ; and so with the garnets, staurolites, and many common species, 

 in the case of other rocks. Again, great beds of the magnesian sili- 

 cate, serpentine, have been made by the alteration of beds of chryso- 

 lite, as in North Carolina, water being the only new ingredient needed. 



3. Origin of Metamorphic changes. 

 The agencies concerned in the metamorphism, influencing chemical 

 and crystallizing forces, are : (1) Heat; (2) water or moisture; and, 

 in some cases, (3) mineral ingredients derived, either in solution or 

 vapor, from outside sources. 



1. Regional Metamorphism. 



1. Temperature. — The heat causing regional metamorphism was, in 

 general, (1) below that of fusion or plasticity ; for most metamorphic 

 rocks, like gneiss and mica schist, have not lost the original bedding. 

 This is shown by the schistose structure, since it usually corresponds 

 with the bedding or the lamination the rock had before its alteration. 

 (See page 794.) 



The heat was (2) sometimes sufficient to produce plasticity. For 

 (a) a passage of gneiss into granite may be often observed, in which 

 the bedding of the former disappears wholly in the course of a few 

 yards or rods ; (b) masses of an associated rock are sometimes in- 

 volved in the granite, showing that the beds had been kneaded to- 

 gether while in the plastic state ; and (c) fissures have been filled by 

 the plastic granite. The passage of gneiss into granite is often thus, 

 as the author has observed at several places in Connecticut : the hard 



