No. 136,] 



33 



believed that they were the original crust of the earth, first 

 formed by the cooling of its melted mass, J but these names are 

 now replaced by Ihose we have given ; for it is doubted whether, 

 if such, a crust exists, we can identify it, and many able geolo- 

 gists think that most of the granite and other hypogene masses 

 are only re-melted and altered forms of stratified rocks. That 

 many such masses are so, is certain ; whether we can find any 

 which are portions of an original crust of the globe, is at least 

 very doubtful. 



Containing no fossils, these rocks have their chief interest from 

 their value for economic uses in building and other purposes, and 

 in the remarkable minerals which they so often contain. Most of 

 the beautiful crystals which form the collections of the mineralo- 

 gist are from these rocks, or from the metamorphic masses which 

 overlie them ; and the latter, where they are seamed with veins 

 or dykes of melted hypogene rock, are often depositories of the 

 metallic ores. 



To recapitulate the order of these three great classes of rocks, 

 beginning yrpm below^ we find : 



1. The Hypogene or Plutonic masses, seen in this State in the 



Adirondack region and the mountainous and hilly south- 

 eastern part of the State. 



2. The Metamorphic, generally lying next above and adjoining 



to the former. 



X The " nebular hypothesis " in astronomy leads us to think it probable that the earth first 

 existed as a hot gaseous or *' nebular " body; that it afterwards slowly condensed by loss of 

 heat into a liquid or melted globe, and that still further cooling during the lapse of agea 

 brought it to a state in which it was covered with a thick rocky crust, and go far cooled that 

 water condensed upon it, when the great series of changes began which have been before 

 explained, growing out of the action of rain and rivers and sea, and leading to the formation 

 of the stratified rocks. Many phenomena seem to confirm this opiniou : such are the many 

 volcanoes yet in activity; the innumerable instances where trap, lava, porphyry, granite, 

 and other similar rocks appear to have risen in a melted state from below through fissures or 

 crevices of the higher rocks; the generally changed or "metamorphic" condition of many 

 of the deepest and oldest stratified rocks; and the universal increase of heat found in deep 

 mines or borings, being on the average a degree for every fifty or sixty feet in depth ; a rate 

 of increase which, continued to thirty or forty miles below the surface, would melt granite or 

 basalt. The form of the earth, also, slightly flattened at the poles and bulging at the equa- 

 tor, is precisely that which a fluid globe rotating on its axis would assume. It thus seems 

 probable that the globe of our earth is, at a moderate depth, in a state of fusion or intense 

 heat, covered only by a crust thick enough to prevent further radiation of heat to any sen- 

 sible degree. Yet there are some reasons for doubting this theory ; and it can yet be regarded 

 only as an unproved, though very probable hypothesis. It is so often mentioned or alludtd 

 to, and seems to afford so good an explanation of some phenomena in geology, tht^^ this brief 

 mention of it is deemed proper. 



[Assembly, No, 136.] 



