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NEW YORK STATE MUSEUM 



either changed sedimentary or igneous rocks, but are regarded 

 somewhat doubtfully as being of igneous origin. 1 



None of the old water-deposited rocks occur within the limits 

 of the sheet, the oldest rocks found being gneisses of uncertain 

 origin, in Dannemora and Beekmantown. After their formation 

 and when they were essentially in their present condition the 

 mass of igneous rock now constituting the gabbro mass of Rand 

 hill was intruded into them from below. At the time of this 

 intrusion the rocks now at the surface were at a depth of at least 

 5 miles, that is they had at least that thickness of rocks above 

 them, and the igneous rock solidified at that depth. At some 

 time after solidification all the rocks were subjected to great 

 compression, and the massive igneous rock was converted into 

 a gneissoid rock. A long period of erosion followed, hence the 

 region must have been above sea level at this time. The result 

 of this slow but long-continued wearing away of the surface 

 was to decrease the depth at which the present surface rocks 

 lay buried. Another period of deformation followed and the 

 rocks were fissured, sheared and probably faulted, the difference 

 in the character of the deformation being due to the fact that 

 the rocks were less deeply buried and hence under less load than 

 during the previous deformation period. 2 The main set of 

 fissures so produced had an east and west trend and a nearly or 

 quite vertical inclination. During or closely following this 

 period igneous rock again worked its way up from below, follow- 

 ing these east and west fissures. Whether any of this molten 

 rock ever reached the surface or not can not now be told, as no 



Crystalline rocks in which there is a more or less parallel arrangement 

 of the constituent minerals, due to which there is a tendency to split or 

 cleave in one direction into more or less even slabs or layers, are said 

 to have a foliated structure. The splitting is usually due to the presence 

 and parallel arrangement of some cleavable mineral such as mica. The 

 foliation is better and more even in schists than in gneisses. 



•At a sufficiently great depth below the surface the pressure due to 

 the weight of the rocks above is so great that the formation of cracks 

 or cavities is impossible. The necessary depth varies with the kind of 

 rock, but at 5 or 6 miles all fracturing is impossible, that is the forma- 

 tion of open fractures. Hence the character of the effects produced by 

 deformation is an indication of the depth at the time of deformation. 



