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160 H. r. GUSHING rALEOZOIC IN NORTH-WESTERN NEW YORK 



The sandstone is mostlj' yellow to brown in color, but some is white, 

 and near the base there is locally much red and banded red and white 

 rock. The upper portion is more thinly and regularly bedded than the 

 lower; cross-bedding is frequent. The lower portion is very massive and 

 very irregularly bedded, with an obvious tendency to l)e influenced by 

 the irregularities of the floor. 



Toward the summit the rock becomes weaker and more friable, with 

 frequent brown spots. The cement is in part calcareous and the brown 

 spots seem due to the leaching out of the calcareous matter of former 

 fossils. The summit bed is a thick but weak sandstone of this type and 

 is full of scolithus burroAvs. In these upper beds linguloid shells occur 

 in considerable abundance, though no other identifiable fossils have been 

 seen. A large linguloid form, whose specific determination is not yet 

 settled, is the most abundant. 



In a few localities in the district small patches of no great thickness 

 of a red sandstone are found resting on the pre-Cambrian rocks, which is 

 much more thoroiighly indurated than the normal rock and precisely 

 resembles the red rock of the quarries of the type locality at Potsdam. 

 Many pebbles of this rock are found in the apparent basal conglomerates 

 of the formation here and there in the district and suggest an uncon- 

 formity within the formation. Certainly a pause in deposition is indi- 

 cated with induration and wear of the red rock, but with as yet no evi- 

 dence as to the length of time concerned. 



THERESA DOLOMITE 



The Potsdam grades upward into a formation which consists of sandy 

 dolomite layers and beds of weak brown sandstone which are mostly near 

 the base and are quite like the upper beds of the Potsdam. The line 

 between the two formations is draAvn at the base of the first dolomite 

 layer, but in all probability this is not a constant horizon over the dis- 

 trict. The name given to the formation is mainly intended for local use 

 and the necessity for its introduction arises from present doubt as to the 

 exact equivalent of the formation elsewhere in the region. 



When fresh the dolomite is a hard and tough, bluish gray rock, which, 

 however, quickly weathers to iron-stained sandy crusts. All the beds 

 are somewhat sandy. The most characteristic lithologic feature of the 

 rock is the glittering calcite cleavages which appear on the freshly frac- 

 tured surface. These cleavages run up to an inch in length, have a 

 somewhat satiny luster, owing to the included sand grains, and are 

 produced by the deposit of the calcite cement around tlie sand grains 

 with similar crystallographie orientation; in other words, producing true 



