QUARRY MATERIALS OF NEW YORK 13 



The igneous and metamorphic rocks are distinguished from the 

 sedimentary by their crystalline character, the minerals of hoth 

 having crystallized within the mass. The two are closely associated 

 in areal distribution and together make up the oldest land surfaces 

 now exposed to view. The great Adirondack highland consists 

 entirely of their representatives, all antedating the earliest of the 

 sedimentary rocks that lie upon its border and that- in fact have 

 been derived from the disintegration and erosion of the crystallines. 



The structure and appearance of the different groups are con- 

 ditioned by the agencies which have operated in their formation. 

 These features can be best explained, therefore, in the light of the 

 physical and chemical processes now effective within the earth and 

 that have been in force probably since primitive geological times. 

 The general scientific conception of the earth is that of a cooling 

 body, with the interior in a highly heated state, sufficiently hot to 

 produce instant fusion on release of the load of overlying rocks. 

 If the earth was once thoroughly molten, as is postulated by most 

 geologists, then the cooling process must have led to the formation 

 of an igneous crust in the first instance. This primitive crust, 

 through the attack of waters which settled upon it and the decom- 

 posing effects of the gases of the atmosphere, afforded the source 

 of the earliest sediments, which were deposited in the depressed 

 portions occupied by the seas. There are no known representatives 

 at present of these earliest igneous and sedimentary formations. 



The conditions of cooling, however, must produce a continuous 

 source of strain within the earth in the effort of the outer portion 

 to adjust itself to the still shrinking interior. The periodic release 

 of this strain is evidenced in the production of faults and folds 

 within the crust, affording the relief of pressure necessary for the 

 liquefaction of the potentially molten rock in the interior and its 

 migration toward the surface. Igneous activity, consequently, has 

 not died out, but is still manifest in volcanoes and may be in progress 

 in the hidden depths through the slow movement of large bodies 

 that never reach the surface. 



It is also believed that crustal adjustments take place in conse- 

 quence of the shifting of load upon the the superstructure through 

 the work of rivers. The large rivers bear immense amounts of 

 detritus to be deposited in the seas hundreds and even thousands of 

 miles from the sources. The continental interiors are being worn 

 down and the coastal plains built up in this way. The change of 



