SIXTH REPORT OF THE DIRECTOR IO/X) IO7 



Trenton limestone; with usually a small thickness of Black River 

 limestone beneath 



Unconformity 



Little Falls dolomite; light gray to dark gray, crystalline to 

 subcrystalline dolomite, sharply bounded rhombs embedded in 

 a cement which is more or less calcareous and dissolves away 

 leaving surfaces of sandy appearance ; certain layers are full 

 of nodules of crystalline calcite, others are drusy and hold 

 calcite and quartz crystals ; black and gray cherts are frequent 

 at certain horizons ; a Cryptozoon reef occurs in the upper 

 part of the formation in a dove limestone band; otherwise fos- 

 sils are very scarce. Thickness at least 150 feet and possibly 

 200 feet 



Hoyt limestone member; blackish, subcrystalline, pure or only 

 slightly magnesian limestone alternating with beds of blue and 

 light gray dolomite; quartz sand grains in some of the beds, 

 increasing in amount below ; in the lower portion beds of calcareous 

 sandstone and more rarely of quartzose sandstone; contains 

 many beds of black oolite, most abundant near the base ; con^ 

 tains Cryptozoon, trilobites, gastropods, and L i n g u 1 e p i s 

 acuminata at many horizons. Thickness 80-120 feet 



Passage beds from the Hoyt limestone to the Potsdam sand- 

 stone; alternating beds of hard, vitreous sandstone, gray, cal- 

 careous sandstone, blue or gray, crystalline dolomites and 

 magnesian limestones, and black, oolitic beds ; contain trilo- 

 bites and Lingulepis acuminata. Thickness 40-60 

 feet 



Potsdam sandstone; light colored, vitreous sandstone, with occa- 

 sional layers of calcareous sandstone in the upper portion, and 

 more or less coarsely conglomeratic beds at base; thickness 

 variable because of overlap on an irregular erosion surface of 

 Precambric rock; from 60 feet to more than 200 feet 



So far then as can be told from the surface exposures the 

 thickness of the beds between the Potsdam and the Mohawkian 

 is from 300 to 350 feet. 



The most complete and continuous section of the upper beds 

 of the Little Falls dolomite seen is that along the roadside east 

 of Highland Park. Here the following section was measured. 



