326 DYNAMICAL GEOLOGY. 



where a pure limestoue is upturned at a high angle, — this position being 

 evidence of its subjection at some time to heavy pressure. 



Tlie heat for the changes in granitic veins, like the Branchville, may 

 have been produced by friction from an up or down movement along the 

 vein ; and the same is probably true for the bed of iron ore at Brewster, in 

 eastern New York ; for veins, and also ore-beds when they are nearly vertical, 

 are planes of weakness. But whether the movement occurred at the epoch 

 of mountain-making at the close of the Lower Silurian, or at some other simi- 

 lar epoch, is unknown. 



Relations of metamorphic and igneous rocks. — The earth's interior source 

 of heat has had much to do in geological history with metamorphism as well 

 as with igneous ejections. The depth to the region beneath the earth's 

 surface having a temperature near the fusing point of the rocks has 

 increased with the progress of the geological eras; the amount of meta- 

 morphism has correspondingly decreased through the ages. 



In early Archaean time the region of fusion was at the surface, and in 

 the later part, before solidification was complete, it was not far below the 

 surface. Great stratified formations had then already been made — 30,000 

 feet in thickness at least, and some have said twice this, or more. A 

 temperature close to that of fusion may then have been within this pile of 

 deposits (page 258, paragraph c), so that but little addition to the heat from 

 subterranean movements would have produced not only ordinary metamor- 

 phic effects, but also fusion of portions of the sediments, making granite, 

 gabbro,and other igneous rocks. 



Metamorphic work was extensively carried on at the close of the Lower 

 Silurian in eastern North America, and igneous rocks were among the 

 metamorphic results ; it was much less extensive at the close of Paleozoic 

 time, and later than this it is not known to have occurred. In western 

 North America, in California, however, the results of heat were large even 

 in the later part of Mesozoic time. We may account for this difference 

 between the two sides of the continent, perhaps, by the fact that the Pacific 

 border had already become a region of extensive volcanic action, — evidence 

 that the depth to great heat was unusually small. 



On the contrary, volcanic action has increased through the ages. There 

 is no good reason for believing that there was much volcanic or deep-seated 

 igneous action in Archaean time. The earth had then its granites, its gabbros, 

 its syenytes, and other igneous rocks ; but no petrological study can show 

 whether the fusion was among the results of metamorphic action or not. 



In this connection it is an instructive fact that in eastern North America, 

 at epochs when there was the greatest amount of friction and crushing, — 

 those of the making of the Green Mountains and Appalachians, — no vol- 

 canoes were made, and little took place in the way of eruptions through 

 fissures ; the conditions were largely those of the past. But at an epoch in 

 Mesozoic time, when there was almost no dexing of the rocks, and only low 

 monoclinal uplifts, extensive dolerytic eruptions occurred at intervals for 

 1000 miles along the Atlantic boiaer. 



