METAMORPHIC ROCKS. 219 



type. The remainder of the fused mass after long standing would 

 separate into a lighter acid portion above and a heavier basic portion 

 below. These would, therefore, be successively erupted as rhyolite and 

 basalt. 



CHAPTEK IV. 

 METAMORPHIC ROCKS. 



There is a third class of rocks, intermediate in character between 

 the ordinary sedimentary and the igneous rocks, and therefore put off 

 until these had been described. The rocks of this class are stratified, 

 like the sedimentary, but crystalline, though never glassy, and usually 

 non-fossiliferous, like the igneous rocks. They graduate insensibly on 

 the one hand into the true unchanged sediment, and on the other into 

 true igneous rocks. 



Origin. — Their origin is evidently sedimentary, like other stratified 

 rocks, but they have been subsequently subjected to heat and other 

 agents which have changed their structure, sometimes entirely destroy- 

 ing their fossils and even their lamination structure, and inducing in- 

 stead a crystalline structure. The evidence of their sedimentary origin 

 is found in their gradation into unchanged fossiliferous strata ; the 

 evidence of their subsequent change by heat, in their gradation into 

 true igneous rocks. For this reason they are called metamorphic 

 rocks. 



Position. — All the lowest and oldest rocks are metamorphic. The 

 converse, however, viz., that metamorphic rocks are always among the 

 oldest, is by no means true. Metamorphism is not, therefore, a test of 

 age. Metamorphic rocks are found of all ages up to the Tertiary. The 

 Coast Eange of California is much of it metamorphic, although the 

 strata belong to the Tertiary and Cretaceous periods. Metamorphism 

 seems to be universal in the Laurentian, is general in the Palaeozoic, 

 frequent in the Mesozoic, exceptional in the Tertiary, and entirely 

 wanting in recent sediments. It is therefore less and less common as 

 we pass up the series of rocks. The date of metamorphism is also 

 different from that of the origin of the strata. Metamorphism has 

 taken place in all geological periods, and is doubtless now progressing 

 in deeply-buried strata. 



Metamorphism is also generally associated with foldings, tiltings, in- 

 tersecting dikes, and other evidences of igneous agency, and is there- 

 fore chiefly found in mountainous regions. It is also usually found 

 only in very thick strata. 



Extent on the Earth-Surface. — These rocks exist, outcropping on 



