﻿GEOLOGY OF THOUSAND ISLANDS REGION 20, 



calcite. Toward the edges, however, the rock becomes much less 

 pure, and at times the same thing happens in the near vicinity of 

 the igneous intrusions, large and small, which repeatedly cut through 

 it. This is by no means the invariable rule however. In the case 

 of the thin limestone bands which occur in the general schist series 

 [pi. 2] there is much less pure limestone, since these bands show 

 the same impure borders as does the thick belt, leaving only a 

 small central thickness of the purer rock. In this pure rock oc- 

 casional graphite scales, flakes of brown mica (phlogopite) and oc- 

 casional small crystals of white pyroxene (diopside) are the usual 

 accessory minerals and in very small amount. Others occur, but 

 very sparingly. These rocks must originally have been extremely 

 pure limestones, slightly contaminated with organic matter, which 

 now appears in the form of graphite. 



The impure limestone of the area is owing to two distinct causes. 

 Certain thin bands of impure limestone in the schist series, and the 

 impure borders of the otherwise pure bands seem unquestionably 

 owing to original deposit as shaly or sandy limestones, forming 

 gradations between the pure rock and the overlying and underlying 

 shales and sandstones. Hence on recrystallization a much smaller 

 percentage of calcite and a much larger one of other minerals has 

 resulted. The other cause is the interaction of the limestone with 

 igneous rocks, producing what are known as contact rocks, in which 

 certain added ingredients are supplied to the limestones from the 

 igneous rocks and react with the limestone to form minerals which 

 thus have a mixed origin. Such contact rocks are thus limited to 

 the near vicinity of the igneous rocks. 



The two most common kinds of impure limestone of the first 

 type in the region are the quartzose limestones, and the pyroxenic 

 limestones. Much of the marginal limestone seems to have been 

 sandy, and even to have contained thin layers of fairly pure sand- 

 stone. This has recrystallized as quartz, partly in fine grain, form- 

 ing a mosaic with the calcite, and partly coarser and in films and 

 patches in the limestone. Each mineral at times contains inclu- 

 sions of the other, they evidently recrystallized together, and the 

 quartz evidently had the stronger crystallizing force. There is a 

 considerable amount of limestone in the area which is a calcite- 

 quartz rock, with little or no admixture with other minerals. 



Even more common is the pyroxenic limestone, where the cal- 

 cite is accompanied by a greater or less amount of a white or a 

 light green pyroxene. This is prone to alter to serpentine, a dull 

 green, greasy to earthy looking mineral, producing a mottled green 



