GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



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the uncertain gneisses involved often with the Grenville rocks, 

 yet without any Grenville admixture, and the relationship of such 

 rocks forms a very difficult problem. The Grenville belts and 

 patches and the areas occupied by the later igneous intrusions 

 have been in the main discovered and mapped. There yet remains 

 the exceedingly difficult problem of the separation of these mixed 

 belts into their several elements, and the working out of their 

 affiliations. This is likely in many cases to prove impossible, and 

 in nearly all cases the amount of intermingling is so great as to 

 render attempts at detailed mapping of the several elements futile, 

 and to require their designation as belts of mixed rocks. 



Grenville rocks. The most characteristic of these are the lime- 

 stones. They are always thoroughly, and usually coarsely crys- 

 talline marbles, which even when purest contain scales of 

 graphite. They vary greatly in purity and usually contain green 

 and white pyroxenes, apatite, phlogopite, quartz and scapolite, 

 often in large quantity. In the thick beds these are more apt to 

 be concentrated in the outer portions, in fact the limestone often 

 grades into a pyroxene quartz rock, with or without scapolite, or 

 else into a nearly pure pyroxene rock. Some beds of apparently 

 pure limestone are found to contain a large quantity of white 

 pyroxene, and when this has altered to serpentine, as it tends to 

 do, the white and green mottled, calcite serpentine rock known 

 as ophicalcite results. 



There is always found associated with the limestones a series of 

 curious schists and gneisses, often found also where no limestone 

 is present, which are difficult to describe, owing to their many 

 phases, but which are easy of recognition and are as characteristic 

 of the series as are the limestones. They vary from exceedingly 

 quartzose to quite basic rocks. Garnet, graphite, sillimanite, 

 pyrite and white pyroxene are very frequent and characteristic 

 minerals. Many of the beds, specially those which contain pyrite, 

 weather readily to a peculiar, rusty looking rock, seemingly much 

 more altered than is actually the case. Many others are exceed- 

 ingly quartzose, so much so as strongly to resemble quartzites, but 

 these are found to contain usually much alkali feldspar, rocks that 



