GEOLOGY OF OGDENSBURG I? 



places associated with the limestones, on the Ogdensburg quad- 

 rangle, but the bands are quite too thin to map separately on this 

 scale. 



Associated with the limestones, especially as marginal phases, are 

 dark-colored, heavy gneisses, composed largely of pyroxene,' usually 

 with much graphite also, which apparently represent rather impure 

 limestones. These rocks, when heavily metamorphosed, lose their 

 carbon dioxide to a large extent, and the lime combines instead with 

 the silica present to form various lime silicates, mostly pyroxene. 



IGNEOUS ROCKS 



Three different varieties of igneous rocks have been mapped on 

 the quadrangle, namely, granite, syenite and diabase. The rock 

 mapped as syenite is really a granite, but because it differs markedly 

 from the general granite of the region and is precisely like rock 

 which elsewhere occurs as a marginal phase of the general syenite 

 of the region, it is so mapped here. It occurs in long, narrow 

 tongues which may be dikelike projections from a larger syenite 

 body to the south on the Gouverneur sheet which is not yet mapped ; 

 or there may possibly be such a body to the north, under the 

 Paleozoic rock cover. 



A fourth variety of igneous rock should be added, namely, the 

 solid variety of the amphibolite, already described, which is 

 regarded as probably a metamorphosed gabbro. 



Granite. Two considerable areas of granite-gneiss are exposed 

 within the mapped district, the one in Dekalb, in the southeast 

 corner of the Ogdensburg. sheet, and the other in Macomb, in the 

 corresponding corner of the Brier Hill sheet. A third small area 

 is shown in Hammond, on the south margin of the Brier Hill sheet, 

 and this is of interest as being the extreme northeast prolongation 

 of the granite mass described as the Alexandria bathylith, in the 

 description of the geology of the Alexandria quadrangle in the 

 Thousand Islands report. 1 From Chippewa bay this granite 

 extends to the northeast, up the valley of Chippewa creek, flanked 

 by Potsdam sandstone on both sides, across the northwest corner 

 of the Hammond sheet to the Brier Hill sheet, before the adjacent 

 Potsdam closes around it from the sides and hides its further 

 extent. It shows on Brier Hill in but a single outcrop, but the 

 particular interest which it has for us here is that it is a part of 



1 N. Y. State Mus. Bui. 145, P- 36-38. 



