34 NEW YORK STATE MUSEUM 



may belong to that very ancient rock floor. Again, the fact that 

 Grenville sediments were being deposited under water carries with 

 it the corollary that there must have been land somewhere at no 

 great distance from the area of deposition because, then as now, 

 such sediments as muds and sands could have been derived only 

 from the erosion or wear of land and have been deposited in layers 

 under water adjacent to the land mass. Here too we are as yet 

 utterly in the dark regarding any knowledge of the location or 

 character of that very ancient land. 



Perhaps the most interesting and characteristic of the Grenville 

 rocks is the crystalline limestone or marble. It is widely scattered 

 in small to large areas throughout the Adirondacks. Typically it 

 is a white, coarse-grained, granular rock made up almost entirely 

 of crystals of calcite.^ When exposed to the weather it often 

 crumbles to a gravelly looking mass. Layers of dark or greenish 

 rocks often occur within the limestone, these representing what were 

 originally layers of sea mud interstratified with the lime. Black, 

 shiny flakes of graphite (so-called " black lead ") may nearly always 

 be found in the limestone, while quartz grains several millimeters 

 across are often abundant in certain portions of the limestone for- 

 mation. This limestone could not possibly have been of other than 

 sedimentary origin. 



In numerous places, and sometimes in sharp contact with the 

 limestone, are beds of almost pure quartz rock or so-called qnartsite. 

 Such rock is a crystallized, metamorphosed sandstone and could 

 not possibly have been of igneous origin. This quartzite is 

 generally in thin layers of clear flintlike or glassy appearance and 

 with perfect stratification. Sometimes other minerals, especially 

 white mica (muscovite), are mixed with the quartz. 



Most abundant of all the Grenville rocks, however, are very 

 extensive, thick deposits of usually dark to light gray rocks rich 

 in such minerals as quartz, feldspar, garnet, mica, pyroxene and 

 amphibole. Because of their distinct banded (stratified) structure 

 and close association (often interbanded) with the limestone and 

 quartzite, they are also certainly ancient sediments, in this case 

 original sea muds or less admixed with sand and lime. As a 

 result of heat, pressure and moisture, these sea muds have been 

 crystallized into what are now called schists or gneisses (see page 

 lo). Certain of the quartzites, schists and gneisses also at times 

 contain scattering flakes of graphite ("black lead"), in some cases 



1 See appendix for descriptions of common Adirondack minerals. 



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