452 



HISTORICAL GEOLOGY. 



and (3, 4a, 4&) overlying Lower Silurian strata. Such sections of Cambrian 

 strata over the upturned Archaean are proof that the mountain-making in 

 the region preceded the Cambrian era. It is probable that the Adirondacks 

 were made at the close of Archaean time. They were, from the first, great 

 mountains, for the highest of the summits, Mount Marcy, now stands 5000 feet 

 above the Cambrian seashore, or the lowest Cambrian beds, and this is the 

 height remaining after long ages of denudation. For the original height, 

 8000 feet above the Cambrian tide-level can hardly be too high an estimate. 



502. 



From the south side of the St. Lawrence in Canada, between Cascade Point and St. Louis Rapids: 1, gneiss; 

 2, overlying Potsdam sandstone; 3, calciferous sand -rock ; 4a, Trenton limestone; 46, Hudson slates. 

 Logan. 



The fusion of beds by the heat in the lower and hotter part of the geo- 

 syncline would have made, by the escape of the liquid rock alone, fissures, 

 veins of igneous rock in the metamorphic region, and also inclosures of the 

 broken schists of the upper and less heated part of the mass (page 448). 

 Such igneous eruptions are of the same age as the metamorphism. 



How many epochs of upturning occurred in the course of Archaean time 

 is unknown. In the vicinity of lakes Huron and Superior (and probably 

 also farther east) there was one at the close of the Laurentian period. 



Over the Archaean area of New Jersey, and of Orange and Putnam counties in New 

 York, there are several long belts of Cambro-Silurian rocks, occupying what were 

 originally valleys of Archaean time, having the northeastward trend of the rocks. They 

 are fossiliferous in New Jersey, and partly metamorphic in Putnam County, N.Y., north 

 of Peekskill. They once spread more widely over the Archaean Highlands, and, perhaps, 

 covered the whole when the Coal-measures were finished, as considered probable by 

 J. P. Lesley. The upturning the beds have undergone took place in spite of resistance 

 to fracture or compression in the underlying Archaean rocks. 



SUBSEQUENT ALTERATIONS OF ARCH^AN ROCKS. 



Archaean rocks have in many places undergone changes in their minerals. 

 They were made at higher temperatures, under greater atmospheric pressures, 

 and with slower rates of cooling, than ordinarily obtain now at the earth's 

 surface ; and these changed conditions, and especially those due to heat from 

 orographic movements, have occasioned alterations in some constituents. 



Many Archaean rocks that are now hornblendic were originally pyroxenic. Since 

 other pyroxene rocks have remained unchanged, some circumstances must have intervened 

 to commence the alteration ; and it may be that it was a heating up of the rocks to 1000° F., 

 through fracturings, faultings, and crushings attending earth-movements or mountain- 

 making. Besides the above-mentioned change, chrysolite, pyroxene, hornblende, and 



