﻿GEOLOGY OF THOUSAND ISLANDS REGION 63 



the Grenville limestone were filled with this type of sandstone, and 

 it occurs frequently as pebbles in the otherwise basal conglomer- 

 ates, being the only sort of sandstone occurring as pebbles in such 

 situation. The thorough induration seems certainly to have taken 

 place before the pebbles were formed. There seems no way to ac- 

 count for these conditions except to assume that there was an 

 earlier deposit of sand in the district, likely in no great amount, 

 and chiefly in the Grenville hollows, deposition ceased, thorough 

 cementation followed and then erosion; in other words that there 

 was a slight amount of deposition here in earlier Potsdam time, 

 separated by an erosion unconformity from the bulk of the forma- 

 tion. 



Occasional beds of black, and of mottled black and white sand- 

 stone appear in the upper part of the formation. The coloring 

 matter is entirely in the cement, which is silicious, and is wholly 

 discharged at a low red heat, hence likely organic. 



In the uppermost 10 to 15 feet of the formation calcareous ce- 

 ment reappears, foreshadowing the change which gave rise to the 

 overlying dolomite formation. In consequence of this the rock 

 weathers easily to a weakly holding, brownish sand, usually mottled 

 with spots of deeper brown. This portion is mostly thin bedded but 

 terminates above in a very massive layer, 2 feet thick or more, which 

 is comparatively resistant owing to its massiveness ; and this heavy, 

 brown mottled layer often full of small, rounded sand concretions, 

 makes a convenient summit for the formation, owing to its rela- 

 tive prominence. The .first layer of gray dolomite usually comes in 

 directly above, and if not, no more than a foot or two of sand- 

 stone intervenes. . The two formations grade into one another, so 

 that any line of subdivision must be an arbitrary one. We have 

 drawn it at the base of the first dolomite layer to appear, and this 

 closely corresponds with the summit of this thick sandstone. There 

 is, however, some reason for the belief that the base of the upper, 

 calcareous sandstones should be made the division line. 



With the exception of the long trails of an unknown animal, to 

 which the name of Climactichnites has been given, some of which 

 have been found in the sandstone 1 mile west of Theresa, no fossils 

 have been found in the formation in this district except in these 

 upper, calcareous beds. 1 In these a large linguloid shell (iden- 

 tified by Ulrich asLingulepis acuminata) is quite com- 

 mon, and passes up into the lower beds of the Theresa formation. 



iWoodworth, J. B. N. Y. State Pal. Rep't 1902. p. 959-66. 



