32 CALCIFEROUS GROUP. 



5. Massive earthy and silicious limestone containing trilobites, 20 feet, followed 

 by beds of similar character of greater thickness containing brachiopods. 



6. Eed Cystidean limestone, susceptible of a fine polish, 15 feet. 



7. Drab-colored, thin-bedded, earthy magnesian beds, suitable for hydraulic 

 lime, of considerable thickness ; fossils rare, except fucoids. Toward the top of the 

 rock it is blue and frequently cherty, oolitic, and concretionary, the upper masses 

 from 20 to 30 feet thick. 



§ 61. The Group is persistent, and surrounds the irregular dome of Laurentian 

 rocks, which form the northern highlands of New York, in a belt, overlying the 

 Potsdam. It is chiefly a hard calcareous sandstone or arenaceous limestone, resting 

 upon the margin of the Potsdam sandstone, from Lake Ontario eastwardly to Ver- 

 mont, and from New Jersey north, near the line of New York and Vermont, into 

 Canada. It forms a narrow belt of surface exposure, with a variable thickness 

 from 50 to 350 feet. Lake Champlain has cut a channel through it for twenty 

 miles. In Canada, adjacent to New York and Vermont, it is, in the lower part, a 

 dark, bluish-gray, crystalline, strongly coherent dolomite or magnesian limestone, 

 and in the upper part a bluish-gray, calcareous argillite, but its characters are dif- 

 ferent in different localities. It is usually found as a narrow belt following the 

 sinuosities of the Potsdam Sandstone, from west of Lake of the Woods to the At- 

 lantic sea-board ; but where the rocks have been disturbed by volcanic energies it 

 may be absent or difficult of detection. The surface area of its distribution in 

 Canada is several thousand square miles, and in its undisturbed condition the maxi- 

 mum thickness rarely exceeds 450 feet, though in Newfoundland, where it is a 

 definitely stratified limestone, it has a thickness of more than 2,000 feet. In the 

 region of the Mingan Islands, in the Gulf of St. Lawrence, the fossil casts and 

 shells are in a good state of preservation. 



§ 62. In Northern Michigan and on the Menominee and Escanaba it preserves 

 its New York characters in a remarkable degree, although its thickness may not 

 exceed 50 feet. The upper portions are highly calcareous, and on fresh fracture 

 show the peculiar granular structure so characteristic in New York. It is thin- 

 bedded, and contains small cavities lined with crystals of calc-spar, quartz, or horn- 

 blende. The surfaces of the layers are often covered with fucoidal impressions. 

 From St. Mary's River westerly to the Wisconsin and the Mississippi there is a 

 gradual augmentation in the thickness of the rocks and a material change in their 

 composition. The Group enters Wisconsin from Michigan a few miles from Green 

 Bay, and striking south-west upon the border of the Potsdam it forms a serrated 

 margin from five to fifteen miles in width, until it reaches the streams that flow into the 

 Mississippi in the south-western part of the State, where it is exposed upon some of 

 the streams for a distance of 75 or 100 miles. It crosses the Mississippi and the 

 north-eastern corner of Iowa, appearing in the bluffs and hills more conspicuous 

 than the Potsdam, though not so thick, because it is a much firmer rock. It is a 

 buff-colored dolomite, without uniformity of texture or stratification, and weathers 

 into rough, bold, and often picturesque fronts along the valleys, and has a thickness 

 of about 250 feet. 



§ 63. From Iowa the area of exposure is a little west of north through Min- 

 nesota, reaching as far west as the second tier of counties from the Mississippi and 

 following the bluffs with limited outcrops in Wisconsin to Lake Pepin, north of 



