GEOLOGY OF THE PORT LEYDEN QUADRANGLE 9 



river, one branch of which drains the Fulton chain of lakes. 

 Chief among the tributaries from the west are Sugar river. Mill 

 creek, House creek, Whetstone creek and Roaring brook. All 

 the larger streams which have cut across the steep eastern, 

 front of Tug hill have there cut out deep narrow gorges locally 

 called " gulfs." More special physiographic features will be 

 described later. 



PRECAMBRIC ROCKS 



The Precambric rocks of the quadrangle represent a portion 

 of the great Adirondack crystalline mass along its extreme west- 

 ern border. They occupy the eastern side of the quadrangle and 

 make up a little less than one half its area. Except at the 

 extreme south the Paleozoic-Precambric boundary line is every- 

 where to the west of Black river, but it keeps close to the river 

 bottom. These crystalline rocks continue westward, under 

 cover of the Paleozoics, for many miles. Considerable portions 

 of the Precambric area are so deeply buried under glacial drift 

 deposits that it is impossible to gain even the slightest clue as 

 to the character of the rocks in those places. 



Grenville gneiss 



The Grenville formation takes its name from Grenville, 

 Canada. It comprises a series of gneisses representing very 

 ancient sedimentary rocks which have been so profoundly meta- 

 morphosed that the original sedimentary features have been 

 largely obliterated. So far as can be definitely proved they are 

 the oldest exposed rocks in the whole Adirondack region. That 

 they are not actually the oldest rocks is evident from the fact 

 that these sediments must have been deposited upon a still older 

 floor. This very ancient rock floor, which may or may not 

 represent a portion of the earth's primitive crust, has thus far 

 not been proved to exist in the present exposures of the Adiron- 

 dacks. It is barely possible that some of the gneisses still of 

 doubtful age and origin may represent that ancient sea-floor. 



Within the map limits the Grenville has been mapped in only 

 three small areas, one at Kosterville, another at Lyonsdale, and 

 a third to the east of Fowlersville. Grenville rocks are unques- 

 tionably present in much greater force than these small areas 

 seem to indicate but they are always so thoroughly involved 

 with other gneisses that they can not be represented on the geo- 



