METAMORPHISM 343 



occasionally the fragments have been completely absorbed. The 

 metamorphism resulting from intrusions is more extended when the 

 intrusion cuts across strata than when it follows bedding planes, 

 since under the former conditions the effect of the heat is felt along 

 the several bedding planes with which the lava comes in contact 

 (Fig. 336). 



The metamorphic effect of an intrusion is greater than that of 

 an extrusion, since in the former the heat of the magma is lost more 

 slowly, and the neighboring rocks are consequently heated to a higher 

 temperature and for a longer time. Moreover moisture, which is 

 a powerful agent in metamorphism and in the production of crystal- 

 line structure in rocks, is more likely to be present under the former 

 conditions. It frequently happens that the rock underlying a lava flow 

 is so little metamorphosed that no change is visible to the naked eye. 



The effect of gre^at intrusions has already been discussed under 

 Subjacent Masses (p. 327). 



Regional Metamorphism. — Thousands of square miles of the 

 earth's surface are underlain by metamorphic rocks. They occur 

 over large areas in Canada, in the Adirondacks, over the greater part 

 of New England, in the Piedmont region east of the Appalachian 

 Mountains, in a large area south of Lake Superior, and in the Cordil- 

 leras. 



Widespread or regional metamorphism may be brought about in 

 one of two ways. (1) It may result from great igneous intrusions, 

 such as deep-seated batholiths. (The metamorphism of the older 

 rocks of the Laurentian region of Canada seems to have been pro- 

 duced largely in this way.) (2) Great lateral pressure may also pro- 

 duce sufficient heat to recrystallize the rocks affected. In regions 

 where igneous intrusions are absent, as in New England, the meta- 

 morphism appears to have been caused by lateral pressure alone. 

 The fact that the rocks of some metamorphic regions are more or 

 less highly folded and that the intensity of the metamorphism is, 

 to some degree, in direct proportion to the intensity of the deforma- 

 tion is offered as proof that the alteration of the rocks in such re- 

 gions was due, either directly or indirectly, to the cause or causes 

 which produced the folding. The indirect cause is believed to have 

 been the pressure which produced the deformation ; the direct causes, 

 the heat resulting from the rock mashing produced by pressure, and 

 the presence of underground water which aided powerfully in bringing 

 about the molecular changes which resulted in the crystalline texture. 



