754 DYNAMICAL GEOLOGY. 



4. METAMORPHISM. 

 1. General Characteristics. 



Metamorphism means alteration in rocks ; alteration as to texture, 

 and often also as to mineral constitution ; alteration affecting strata 

 over wide regions at the same time. The sedimentary rocks described 

 on pages 67-69, as sandstones, shales, conglomerates, even coarse 

 conglomerates, and limestones, are the rocks that have thus been 

 changed ; and the crystalline kinds, called metamorphic rocks, de- 

 scribed on pages 70-75, for example, argillyte or roofing slate, mica 

 schist, gneiss, much granite, crystalline limestone or marble — are 

 some of the results. But there is no obvious line of division be- 

 tween the unaltered and the altered or metamorphic rocks ; the 

 changes are of all degrees, and consequently the two kinds pass into 

 one another by insensible gradations. 



For a time after geology had begun to be a science, granite, gneiss, 

 mica schist, were called Primary rocks, under the idea that they were 

 the oldest. But now it is known that such rocks, although to a large 

 extent Archaean or Paleozoic, may be of any period of origin. 



Metamorphic rocks may have undergone in whole, or in part, a sec- 

 ond alteration or metamorphism ; for no condition of the earth's ter- 

 ranes is free from liability to change. 



A single metamorphic region, that is, a region of simultaneous met- 

 amorphism, often covers tens of thousands of square miles, and its 

 altered rocks a depth of many thousands of feet. The operation has, 

 therefore, much of the geographic comprehensiveness of mountain- 

 making ; and in fact, it has usually been one of the consequences of 

 the profounder movements of the earth's crust. But while generally 

 thus wide-reaching, it has also occurred in a more limited way ; and 

 many local changes that have taken place are of the same nature with 

 the grander metamorphic changes. 



Some examples of wide regional metamorphism have been described 

 on pages 151-156, 213, and 400; and the facts there related are a 

 proper introduction to the following discussion of this subject. The 

 reason for believing in regional metamorphism is there presented — 

 that the crystallized or metamorphic rocks in some parts of the meta- 

 morphic region, and especially on its borders where the action is less 

 complete, are found to graduate into ordinary sedimentary rocks, and 

 also to contain more or less perfect traces of the fossils that filled many 

 of the beds. Thus, in the case of the Green Mountain region, in which 

 a crystalline limestone, with conformable schists and more or less of 

 quartzyte, extends from Western Connecticut and the adjoining part 

 of New York to and beyond Rutland, Vt., the limestone contains fos- 



