﻿64 NEW YORK STATE MUSEUM 



Theresa and Tribes Hill formations 



These formations, as mapped, consist chiefly of rather thin bedded 

 layers of blue gray, sandy magnesian limestone which are exceed- 

 ingly tough and resistant rocks when fresh, but weather rapidly 

 to an ocherous, rotten stone [pi. 15]. The basal portion, through a 

 thickness of 15 feet, carries frequent beds of weak, calcareous 

 sandstone in alternation with the limestone, the sandstone being 

 identical in character and appearance with that forming the sum- 

 mit of the Potsdam. These form apparent " passage beds " between 

 the sandstone and the limestone above. The overlying beds consist 

 chiefly of magnesian limestone though occasional sand streaks con- 

 tinue throughout, and there is a varying and, in general, consider- 

 able amount of sand in most of the beds. While this tends to have 

 a streaky distribution, it seldom wholly gives out. The sand is 

 chiefly of quartz, certainly 90$ of it consisting of that mineral, but 

 grains of feldspar, mica, magnetite, pyrite, titanite and zircon are 

 also present and all in quite fresh condition. 



All the rock effervesces freely with acid, and the thin section 

 shows this to be chiefly due to the presence of calcite cement, most 

 prominent in the more sandy portions of the rock. A prevailing 

 and highly characteristic feature of the rock is the appearance, on 

 freshly broken surfaces, of lustrous calcite cleavages. These are 

 due to the coarsely crystalline character of the calcite cement, the 

 crystals ranging from J4 i ncn to * i ncn * n length, and inclosing 

 a number of sand grains, so that they are veritable sand crystals. 

 This lithologic peculiarity is a feature of the rock of this horizon 

 across the entire northern border of the Adirondacks. 



As mapped the general thickness of the formation over the 

 district is from 60 to 70 feet, but the thickness is variable. The 

 thickness steadily diminishes to the west and to the south in 

 the same fashion as the Potsdam's. But there are also local 

 variations in thickness which are to be ascribed to wear of its 

 summit during an erosion interval which separated its completed 

 deposition from the beginning accumulation of the succeeding 

 formation. For instance it has a thickness of but 20 feet near 

 the north end of Perch lake (Theresa sheet) though recovering 

 its normal thickness of 60 feet both to the east and to the west; 

 and that the diminution in thickness is because of the wearing 

 away of its upper beds with the production of a shallow valley 

 is shown by the fact that the overlying formations thicken here 

 by the same amount that the Theresa thins, and that the thicken- 



