18 NEW YORK STATE MUSEUM 



of all traces of the original structures which characterize the 

 sedimentary rocks. The argument for their sedimentary origin 

 rests on their composition, mineralogic and chemical, and on their 

 frequent variations in composition, beds of different original 

 character having produced differing metamorphic rocks, whose 

 comparatively sharp junctions look like old bedding planes. 



The most characteristic rocks of the Grenville series are the 

 crystalline limestones, but these have not been found within the 

 map limits. A single large boulder of impure crystalline lime- 

 stone was noted on the surface of the heavy moraine which covers 

 the district occupying the extreme north-central part of the map. 

 As it is a soft and quite easily destroyed rock, these limestone 

 boulders commonly indicate a parent ledge near at hand, here 

 probably to the north, not far beyond the map limits. 



In the absence of limestone, the rocks regarded as characteris- 

 tically Grenville comprise a series of light colored, often white 

 gneisses, very rich in quartz, interbanded with less quartzose 

 rocks of darker color, and often with a very respectable percentage 

 of black minerals, hornblende, black mica and magnetite. Both 

 rocks contain, often in abundance, garnets of somewhat unusual 

 color, a much lighter red than ordinary and with a rather pink 

 tinge. These are more conspicuous in the dark rocks in general, 

 but the light colored ones are seldom without them. They are 

 commonly of about pinhead size but often run larger, specially 

 in the darker rocks, those with diameters of from % to i^ inch 

 being often quite numerous, and even larger ones are to be found. 



Another mineral which is very characteristic of these rocks and 

 strongly indicative of their sedimentary origin, is graphite (black 

 lead). Shining, metallic looking scales of this mineral occur 

 frequently in the darker rocks, usually of sufficient size to be made 

 out by the unaided eye. 



Another characteristic mineral, this time confined mainly to 

 the light colored rocks and only visible under the microscope, is 

 sillimanite. 



As has been said, the light colored rocks consist almost wholly 

 of quartz and feldspar and are rich in quartz. Their composition 



