GEOLOGY OF SARATOGA SPRINGS AND VICINITY 29 



dikes of unmetamorphosed, igneous rock. Their greater youth is 

 shown by the fact that they cut all the other rocks. As all the 

 other rocks have been more or less metamorphosed, and these have 

 not, they are likely considerably younger. They are found in all 

 parts of the region but most abundantly in Clinton county, on the 

 extreme northeast, whence they diminish in number to the south 

 and west. In the central and western Adirondacks they are com- 

 paratively scarce. On the extreme northwest, in the Thousand 

 Islands region, they become again abundant. They are older than 

 the Potsdam sandstone, the oldest Paleozoic rock of the region, 

 and hence are of age intermediate between it and the other Pre- 

 cambric rocks, and likely of late Precambric age. 



There are two chief varieties of these rocks: heavy, black traps, 

 and less dense syenite rocks of red color. The latter have been 

 so far found only in Clinton county, but the trap dikes range 

 throughout the region. They are not particularly abundant in the 

 Saratoga region, but those that do occur make up for their infre- 

 quency by their size. The usual trap dikes of the region range 

 from I foot to 30 feet in thickness. Most of those near Saratoga 

 are from 50 to 100 feet thick, and we have traced some of them 

 for several miles. Thus the dike numbered i upon the Saratoga 

 quadrangle, the one quarried for road metal north of the village, 

 can be followed foot by foot for 2 miles north and south of the 

 quarry, with an average width of from 75 to 100 feet. To the 

 north it runs into low, swampy ground for 2 miles but beyond 

 that and precisely on the trend of this dike we have repeatedly 

 found a huge dike of the same width, for an additional distance 

 of 7 miles more, which we confidently assume to be the same dike. 

 Both to the east and west of this big dike are others which have 

 been traced for several miles, and are 50 feet or more in width. 



These are all ordinarv diabases. Olivine has not been identified 

 with certainty in any of them. They are labradorite, augite, mag- 

 netite rocks with good ophitic structure. All are considerably 

 altered. It is exceptional to find the augite in fresh condition, and 

 much of the feldspar is also altered. In this respect the dikes 

 show sharp contrast with those of the northern Adirondacks where 

 the larger number of the dikes are very fresh. This we attribute 

 to the more vigorous glacial erosion on the north. 



