GEOLOGY OP THE NORTHERN ADIRONDACK REGION 355 



Basal conglomerates are a prominent feature in Clinton county 

 wherever the proper horizon is exposed. For the most part these 

 are not extra coiarse, the larger pebbles seldom exceeding an 

 inch in diameter. The pebbles are prevailingly or exclusively 

 of quartz, derived from the quartz veins of the Precambrian 

 rocks, and are embedded in a coarse sand matrix in which there 

 is a large feldspar, anid considerable magnetite content. Along 

 most of the northern border the general lack of pebbles of the 

 underlying rocks, which are mostly Saranac gneisses, is in- 

 dicative of quite prolonged wear of the material, so that only 

 the extraresistant pebbles of vein quartz origin were sufficiently 

 durable to persist as pebbles. The undecayed character of the 

 feldspar grains of the sands in these conglomerates indicates that 

 all soil and largely weathered rock had already been removed 

 and carried offshore to be deposited, and that the waves were 

 working on tolerably fresh rock, whose grinding to sand had to 

 be performed by water action alone, unaided by any special weak- 

 ness due to previous weathering. 



In some few localities conglomerates indicative of much less 

 vigorous wave action are found. These contain numerous pebbles 

 of the underlying gneisses, often of large size and showing great 

 variation in size, and quartz pebbles are much less conspicuous 

 or lacking. These seem to be purely local deposits laid down in 

 sheltered hollows in the Precambric floor, whose presence is 

 likely due to uneven depth of weathering of the floor rocks. It 

 is in rocks such as these that the pebbles of diabase and syenite 

 porphyry which demonstrate the Prepotsdam age of these dikes, 

 are found. Such conglomeriates are much less resistant rocks 

 than the commoner quartz pebble conglomerates, and present ex- 

 posures usually show them in much disintegrated condition. 



These heavy basal conglomerates are mainly confined to the 

 northern border of the region, extending as far west as eastern 

 St Lawrence county. South of Clinton county, along Lake Cham- 

 plain, their existence is rather problematic, owing to dearth of 

 exposures of the proper horizon, mainly due to faulting. 



An interesting outcrop of basal conglomerate occurs not far 

 west of Keeseville in Clinton county, nestling in an indentation 

 in the eastern edge of the anorthosite gabbro, the actual contact 



