CAMBRIAN AND LOWER ORDOVICIAN. 121 



how much and how large a part of the whole thickness it constitutes, is wholly uncertain. The 

 writer was the first to show this, and it has lately been reaffirmed by Van Ingen. This portion 

 consists in part of coarse basal conglomerates, in part of poorly indurated sand beds of smaU 

 durability, and in smaller part of thoroughly indurated sandstones. It is nearly everyivhere 

 characterized by a considerable feldspar content, in addition to the quartz, and this feldspar 

 is for th'e most part fresh. Considerable magnetite also appears in places, along with grains 

 of garnet and occasional zircons. The rock has therefore an arkose character in this portion, 

 whUe above it is prevaOingly of pure quartz sand. Red is the predominant color of the base, 

 and there is but little white sandstone in it, while above the latter is the prevailing color. As 

 a general proposition, a feldspar content and a prevalence of red beds go together and are 

 certain signs of the basal portion. 



Basal conglomerates are a prominent feature in Clinton County wherever the proper 

 horizon is exposed. For the most part these are not extra coarse, the larger pebbles seldom 

 exceeding an inch in diameter. The pebbles are prevailingly or exclusively of quartz, derived 

 from the quartz veins of the pre-Cambrian rocks, and are embedded in a coarse sand matrix 

 in which there is a large feldspar and considerable magnetite content. Along most of the 

 northern border the general lack of pebbles of the underlying rocks, which are mostly Saranac 

 gneisses, is indicative of quite prolonged wear of the material, so that only the extra-resistant 

 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 weakness due to previous weathering. 



In some few locahties 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 hoUows in the pre-Cambric 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 pre-Potsdam age of these 

 dikes are found. Such conglomerates are much less resistant rocks than the commoner quartz- 

 pebble conglomerates, and present exposures usually show them in much disintegrated condition. 



Very abundant also in the basal portion of the formation are beds of rapidly disintegrating 

 very red coarse arkose sandstones, made up mainly of quartz and feldspar grains and the whole 

 much permeated with red hematite. 



********* 



Well-indurated red sandstones, such as those from the type locahty at Potsdam, are not 

 infrequent in the basal portion of the formation and are numerously exposed at various locah- 

 ties on the north border of the Adirondacks in such situation as to indicate clearly their horizon. 

 At Potsdam itself the section is comphcated by faulting, and the horizon of the red sandstone 

 there can not be demonstrated, though inferentiaUy it is low in the formation. Along with the 

 red there is much hard glassy brown sandstone, also containing fresh feldspars but lacking .the 

 hematite coloration of the red beds. Above, the reds become striped and mottled with white 

 forming a species of passage beds to the middle division. 



Van Ingen is the only observer who has undertaken to differentiate between the middle 

 and upper portions of the Potsdam. He says: 



"The middle portion of the sandstone is made up of well-sorted materials, of finer grain, 

 compactly cemented, and of white, steel-gray, or yellowish color, with very little or no feld- 

 spathic content. The grains of sand are both angular and rounded, with the former predomi- 

 nating. The layers are more regular, though their surfaces are ripple-marked, and in section 

 they are seen to be almost universaUy cross-bedded. Pebbles are found on the surfaces of 

 some layers of the middle portion, but unlike those of the upper portion they seem to have been 



