540 



Report of the State Geologist. 



Series III. At Petersburgh, showing well just north of the road, is 

 one of the largest of the basic gabbro bands that has been met with in the 

 county. Measured across the strike the exposure is fifty yards wade, and 

 the full width is not shown, but within a few yards distance, on both sides, 

 ordinary red gneiss conies in. A few yards south of the river at Petersburgh 

 bridge, is a small exposure of similar gabbro, and half way between Russia 

 and Redford, is another with red gneiss close at hand. On the True brook 

 road, just before reaching the fourth bridge (the road crosses the Brook 

 several times), is still another, which looks like an enormous dike ten yards 

 wide, enclosed in the gneiss. 



These four gabbros are all alike in mineralogic composition, being made 

 up of augite, hypersthene, hornblende, biotite, magnetite, plagioclase (some 

 untwinned) and apatite. Unlike most of the gabbro of the region, they 

 totally lack garnet. The gabbro south of the river at Petersburgh has the 

 ophitic structure, with the large characteristic feldspars and augites, which 

 always occur when that structure is preserved. The others are all granular, 

 and have suffered reerystallization ; that betw een Russia and Redford having 

 the very finely granular structure of the "shear-zone" rocks. The Peters- 

 burgh exposures are readily recognizable in the field as gabbros, while that 

 on True brook more resembles the hornblende gneisses. 



Series IV. The Hardscrabble district, a rather elevated plain between 

 the Saranac and Salmon rivers, which lies partly in southeastern Saranac, 

 is occupied by the Potsdam sandstone, but the heavy drift renders it impos- 

 sible t<> accurately map its southern, western and northern boundaries. The 

 exposures seldom show more than the upper surface. The rock is thin-bedded 

 for the most part, commonly buff, but with some red layers: is coarse and 

 gritty, but seldom pebbly, and most of it disintegrates quite rapidly, feu- 

 well indurated layers showing. 



Series V. Thirteen dikes have been found in the township, most of 

 which are decomposed diabases. No. 41, just west of the third bridge over 

 True brook, is a typical olivine diabase, with large olivine and augite pheno- 

 crysts, the latter of a light rose color and slight pleochroism. The rock is 

 quite fresh, much of the olivine being perfectly sound. 



No. .">•_' is the only bostonite met with. It cuts the gneiss just east of 

 Saranac village, and is a aon-porphyritic, dark red rock, which contains a 

 considerable amount of green biotite and of magnetite. 



Series VI. The sand deposits of the Saranac valley run up the river 

 to beyond Redford, and are very conspicuous at that point, where they have 



