176 PALAEOZOIC TIME — LOWER SILURIAN. 



to 7000 feet thick (Logan). They consist of hlack and blue shales, gray sand 

 stone, with some conglomerate, beds of magnesian and common limestone 

 (some of them fossiliferous), slates containing graptolites, red and green shales 

 of great thickness, and, at the top of the series, a sandstone 2000 feet thick, 

 called the " Sillery" sandstone. Hunt has found some of the dolomitic conglo- 

 merate to consist largely of green-sand (glauconite). 



The rocks east of the Hudson, called Taconic rocks by Professor Emmona 

 (from the Taconic range lying along the western slope of the Green Mountains)^ 

 have been referred to this epoch. (Hunt.) They consist of slates, quartz-rock, and 

 limestone, and include the marble of western Massachusetts and Vermont. Fos- 

 sils, probably of the Trenton period, occur in the Vermont limestone (see p. 391). 



Professor Emmons long since pointed out that these rocks were older than the 

 strata on the west side of the river, and, regarding them as pre-Silurian, has 

 called them the Taconic system. He has estimated the thickness at 20,000 feet. 

 The rocks dip at a large angle as the result of great dislocations and folds, and 

 the true thickness is of difficult determination. It is probable that the thick- 

 ness estimated for the Quebec group is nearer the true amount, as has been 

 suggested by T. S. Hunt. The precise line between the Potsdam and Calciferous 

 strata in this Taconic series has not been ascertained. Continuing along the 

 Appalachians into Pennsylvania, we find the limestone strata predominating. 

 H. D. Rogers estimates the sandstone or lower part at much less than 1000 feet, 

 and the limestone portion at 1950 feet in the Kishicoquillas valley and 5400 

 feet in the Nittany valley. 



In eastern Tennessee, also within the Appalachian chain, the earlier part of the 

 epoch is represented by laminated sandstones, some hundreds of feet thick; 

 above this, a magnesian limestone, often oolitic, estimated at 1000 feet in thick- 

 ness, bluish below, grayish at the middle, and gray and cherty above. 



The Calciferous rocks throughout the Appalachians have been greatly dis- 

 turbed. The beds usually lie with their edges to the surface and dipping at a 

 large angle. 



c. Eastern border. — At the Mingan Islands, in the Gulf of St. Lawrence, the 

 Calciferous sandrock occurs overlaid by a white limestone, which is probably 

 referable to the same epoch. Upon the latter rests the Chazy limestone. The 

 white limestone is fossiliferous, and none of the species are identical with known 

 Chazy forms, while several occur in the sandrock below. 



Structural peculiarities. — (a.) The thin lamination of most of 

 the arenaceous beds is an important characteristic. 



(b.) The layers of the Potsdam sandstone in New York, Canada, 

 Michigan, Wisconsin, and elsewhere, are frequently made up of 

 obliquely laminated layers, as in fig. 61 e, p. 93. 



(c.) The layers in Michigan and elsewhere have sometimes the 

 compound character illustrated in fig. 61 /. This figure (by Foster 

 & Whitney) is from the Potsdam sandstone of Lake Superior. 



(d.) Ripple-marks (fig. 62) are common on many of the layers of 

 the Potsdam sandstone in New York, Canada, and the West, and 

 also in sandstones of the Calciferous epoch in Missouri. 



