

METAMORPHISM. 761 



Different degrees of moisture present would naturally have deter- 

 mined different metamorphic results. Too dry a heat may have made 

 crystalline limestones or quartzytes of feeble cohesion. An excess of 

 moisture with a low grade of heat may have produced the hydrous 

 rocks, chlorite schist, and hydromica schist ; while with less moisture, 

 hornblende* schist and mica schist would have been formed ; and, in 

 fact, a stratum is sometimes chlorite schist in one part and hornblende 

 schist in another, the constituents, excepting the water, being nearly 

 the same. 



Again, sedimentary beds that differ too little to the eye to have 

 distinct names may make very different rocks through metamorphism. 

 A shale is a shale whether it contain potash or not ; but through 

 metamorphism, it might make a mica schist if potash were present, 

 and could not if potash were absent, but might then become horn- 

 blende schist. A stratum of mica schist often graduates into horn- 

 blende schist (as happens a dozen miles west of New Haven, Conn.) ; 

 and because the mud-bed from which the rocks were made varied a 

 little in its ingredients, as mud-beds do in existing oceans. A sand- 

 stone is a sandstone, even when containing much clay ; but after 

 metamorphism, it may be quartzyte in one case and a quartzose gneiss 

 in another. Much or little oxyd of iron in a shale is little noticed, 

 but in the metamorphosed shale it may lead to a whitish crystalline 

 rock in one case, and a black in the other, with different mineral spe- 

 cies giving the color. 



In some cases the bedding of rocks has been obliterated by meta- 

 morphic action, without their reaching the condition of plasticity, in 

 consequence of a tendency to promiscuous crystallization in the grains 

 of the constituent minerals. This is true, for the most part, of those 

 consisting of hornblende alone (hornblendyte), hornblende and a feld- 

 spar (dioryte, labradioryte), feldspar (felsyte), feldspar and quartz 

 (granulyte or mica-less granite, quartz-felsyte), serpentine, and some 

 others, as explained on page 627. 



A bedded structure may also be obliterated by the soldering together 

 of layers, when the rock is subjected to heavy pressure, and all evi- 

 dence of it disappear, unless the layers differ in color or constitution ; 

 as has happened in the marble of Rutland, and in other cases, where a 

 pure limestone is upturned at a high angle, — this position being evi- 

 dence of its former subjection to heavy pressure. 



The following table presents a general view of the composition of the more common 

 rock-making materials, showing their close similarity. These species are briefly de- 

 scribed on pages 52-58. The names mica and feldspar each include several species : — 



Silica Quartz. 



Silica + magnesia and water Talc. 



Silica -f magnesia and water Serpentine. 



