GEOLOGY OF LAKE PLEASANT QUADRANGLE 29 



lacking a diabasic texture, it is almost exactly like the most typical 

 gabbro of the North Creek quadrangle in composition (see no. 51 

 of table on page 31). The feldspar is largely labradorite. 



The gabbro mass at The Gorge is of particular interest. It ap- 

 pears to be of the nature of a stock rather than a dike, though 

 the excellent exposures are almost wholly confined to the river 

 channel. From the interior of the stock near the mouth of The 

 Gorge, very fresh, moderately gneissoid, medium-grained, dark 

 greenish-gray rock may be obtained. This rock never shows a 

 diabasic texture but it does exhibit that peculiar mottled appear- 

 ance so often seen in the Adirondack gabbros. This mottled appear- 

 ance is due to the irregular distribution of the black hypersthene 

 crystals through the greenish-gray mass of granulated feldspar. 

 No. 49 of the table on page 31 shows the composition. Near the 

 mouth of The Gorge a twenty-foot wide band of amphibolite was 

 noted, and one-quarter of a mile below this a forty-foot wide mass 

 of pink granitic syenite extends across the river with gabbro in 

 close proximity on either side. This pink rock is either a large in- 

 clusion or tongue of country rock (see map) through which the 

 gabbro has broken as proved by the existence of two much smaller, 

 though very distinct, masses of the same pink rock as inclusions 

 in the gabbro. Near its eastern end several small inclusions (a 

 few feet across) of a light brown, basic phase of the syenite may 

 be seen sometimes with sharp boundaries and sometimes showing 

 rapid gradations into the gabbro due to partial melting. In spite 

 of nearly continuous exposures along the river, the western border 

 of the stock is not clearly defined. There is quite certainly a 

 fairly rapid gradation of the gabbro into the basic syenite apparently 

 due to local assimilation by the molten gabbro. 



Just above the mouth of Ninemile creek there are some big 

 ledges of gabbro, these being the only exposures in the area here 

 shown on the map. In texture, structure and composition (see 

 no. 47 of table on page 31) this rock is very similar to that in 

 The Gorge. On the south side this gabbro is seen to be much 

 finer grained and excessively gneissoid but without change in com- 

 position (for example, no. 50 of the table). This finer-grained 

 mass is only an amphibolite phase of the gabbro. The country 

 rock is pink granite, and the gabbro contains some inclusions of this 

 granite in the form of streaks or bands from 2 inches to i foot 

 wide, parallel to the foliation, and often pinching out abruptly. 



The 2 mile long area mapped as gabbro in the valley of the 

 West Branch of the Sacandaga just above Blackbridge presents 



