THE ADIRONDACK MOUNTAINS 33 



The Grenville strata, as Ave see them today, do not look like 

 ordinary sediments such as shales, sandstones and limestones. They 

 have been profoundly changed from their original condition ; that 

 is to say, they have undergone inctauiorpJiisin. The Grenville rocks 

 now exposed to view were formerly buried at least some miles 

 below the earth's surface, the overlying rocks having been removed 

 by erosion through millions of years of time. Far below the earth's 

 siuTace, under conditions of relatively high temperature, pressure 

 and moisture, the materials of the Grenville strata completelv 

 crystallized into various minerals. Rounded, water-worn grains of 

 the original sediments no longer exist, but instead angular grains 

 (crystals) make up the rocks. The original stratification surfaces 

 (that is, surfaces of separation of the layers of sediment) are almost 

 always still present. jNIany times this stratification is very clearly 

 evident where lighter and darker colored layers from a fraction of 

 an inch to several inches or feet wide sharply alternate. This well- 

 defined banded structure is one of the most useful criteria for 

 the recognition of the stratified character of the Grenville rocks. 

 The other Adirondack rocks are all igneous in origin ; that is, they 

 were once molten, and are much more homogeneous throughout 

 large masses, or if they show variation they are never in sharply 

 defined layers. 



Carefvil examination of Grenville specimens freciuently shows 

 that the mineral grains are more or less flattened out parallel to the 

 stratification surfaces. Because of the parallelism of flattened 

 minerals and stratification, and because the Grenville series has 

 never been subjected to severe mountain-making pressure (see 

 below), we are forced to conclude that the mineral flattening took 

 place during the crystallization of the sediments far below the 

 earth's surface under great weight of overlying material and when 

 the strata were still in practically horizontal position. Such mineral 

 flattening would of course have taken place at right angles to the 

 direction of pressure. 



Having established the sedimentary origin of the Grenville series, 

 we are led to the interesting and important conclusion that these 

 oldest known rocks are not the most ancient which ever existed in 

 the Adirondack region. The Grenville sediments must have been 

 deposited, layer upon layer, upon a surface of rtill older rocks. A 

 knowledge of the character and composition of such pre-Grenville 

 rocks would be of very great interest, but thus far we have no 

 positive evidence that such rocks are visible in the Adirondacks, 

 although certain masses still of somewhat doubtful age and origin 



