Invasion to llegional Metamorphism. 1S3 



means of the tension curve. Chemical affinity, revers- 

 ible with change of temperature, is the controlling- con- 

 dition. The temperature limit is sufficiently low so that 

 notwithstanding the abundant presence and high pres- 

 sure of carbon dioxide in the contact zone, silication is 

 the characteristic process. Only on the outer borders 

 does the reaction become reversed. 



Relation of Magmatic Gases to Atmosphere and Ocean. 



It is held by many geologists at the present time that 

 the gaseous and liquid envelopes of the earth are to be 

 looked upon as the differentiated and unconsolidated 

 residues of the igneous rocks. Most commonly, however, 

 they are regarded as having had approximately their 

 present volume since the closing of the formative stages 

 of the earth. The emphasis which is here placed, how- 

 ever, upon the great volumes of gas liberated in igneous 

 invasion, and the very considerable extent of post- 

 Cambrian batholithic invasion only partially revealed by 

 erosion, suggests that the atmosphere and ocean may 

 have received notable additions since the beginning of 

 the Laurentian, and appreciable additions even since the 

 beginning of the Paleozoic. To make a guess in order 

 to give concreteness to this statement: it seems not im- 

 probable that the ocean waters may have increased 25 

 or 50 per cent in volume since the beginning of the 

 Keewatin lava flows, and 5 to 10 per cent since the 

 beginning of the Paleozoic. The nitrogen of the air has 

 perhaps increased equally, with the result that the atmos- 

 phere may now be of a very different constitution than 

 in the earliest geologic ages. 



Limited Depth of the Zone of Anamorphism and Rock Flops'. 



The earth's crust, when under tangential compression, 

 must yield along its zones of greatest weakness. Where 

 geosynclines have permitted the accumulation of sedi- 

 ments, these may reach to depths as great as six or 

 eight miles.. The deeper parts must become condensed, 

 cemented, and hardened; not softened into a sub-moun- 

 tain liquid as was postulated by an older theory. But the 

 shales remain weak and the horizontal bedding is a struc- 

 tural weakness even in the rocks of strongest inherent 

 composition. Geosynclines, then, even without the influ- 

 ence of subjacent igneous action, are zones of weakness. 



