CAMBRIAN AND LOWEE ORDOVICIAN. 123 



nearer the usual figure than the former. These beds are exposed at many localities along the 

 northern border of the region, but seldom suitably for measurement of thickness. They seem 

 usually of considerable bulk. 



Cushing's account of the Potsdam is followed in the report cited by a description 

 of the overlying Beekmantown. In his latest studies, however, he distinguishes 

 under the name "Theresa dolomite" the dolomitic beds which immediately succeed 

 the Potsdam west of the Adirondacks and which he no longer regards as equivalent 

 to the Beekmantown. Gushing ^"^^ says : 



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 drawn at the base 

 of the first dolomite layer, but in all probability this is not a constant horizon over the district. 

 The name given to the formation is mainly intended for local use and the necessity for its intro- 

 duction 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 character- 

 istic lithologic feature of the rock is the glittering calcite cleavages which appear on the freshly 

 fractured 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 

 the sand grains with similar ciystallographic orientation — in other words, producing true sand 

 crystals on a small scale. This same lithologic peculiarity is a feature of the so-called Potsdam- 

 Beekmantown passage beds farther to the eastward, and, so far as my observation goes, is 

 confined to that horizon, not appearing in the Beekmantown above. 



In the lower portion of the formation the large Lingula previously mentioned is abundant, 

 a gastropod which has not yet been determined occurs at all horizons, occasional cystid plates 

 and traces of other fossils are found, and fucoidal markings are frequent on the surfaces of some 

 layers. 



The general thickness of the formation over the quadrangle is from 60 to 70 feet. Like 

 the Potsdam beneath, it thins to the west and apparently also to the south, though widespread 

 sand deposits hide the decisive proof of the latter. In addition to this thinning because of 

 overlap, it has also suffered erosion at its summit, owing to uplift (apparently slight) and wear 

 during a lengthy time interval. For instance, it has a thickness of but 20 feet near the north 

 end of Perch Lake, though recovering its normal thickness of 60 feet within a mile on each side, 

 and the diminution in thickness is by the wearing away of the upper beds. Coincidently with 

 this local thinning, the overlying formation thickens, showing basal beds which are lacking 

 on each side and plainly owing their existence to the surface depression. 



For the description of the overlying Pamelia and higher formations, see Chap- 

 ter IV (p. 185). 



The Potsdam sandstone and possibly the "Theresa dolomite" belong to the 

 Saratogan or Upper Cambrian, formerly called the "Potsdam series." Walcott 

 introduced the term "Potsdam series" in the large sense in 1891, but in 1903 he 

 withdrew it in order to avoid confusion, and introduced Saratogan instead. After 

 giving the reasons for the change he says: *^^ 



The type locality of the Saratogian is north and west of Saratoga Springs, N. Y. The 

 section has, at the base, about 200 feet of evenly bedded, compact grayish to yellowish colored 

 sandstone, that rests unconformably against or upon spurs or ridges of pre-Cambrian gneiss. 

 At a locality 3 miles north of Saratoga Springs the sandstone is about 40 feet in thickness ; it is 



