232 



PALEOZOIC TIME UPPER SILURIAN. 



In the beds at Lewiston there are eight strata : 1, 2, 3, 4, belong to the Medina 

 group, and consist — 1 and 3, of shaly sandstone; 2 and 4, of hard sandstone; 5, a 

 shale, and 6, limestone, are of the Clinton group ; 7, a shale, and 8, limestone, of 



Fig. 357 A. 



Section along the Niagara, from the Falls to Lewiston Heights. 



the Niagara group. The dip is up-stream, as in the figure, but is only fifteen 

 feet to a mile. - — 



Where fullest developed in New York, the Medina group includes four divi- 

 sions, as follow : — 



4. Red marl or shale, and shaly sandstone, resembling No. 2, below ; banded, 

 and spotted with red and green. 



3. Flagstone, — a gray, laminated quartzose sandstone, called " gray band." 



2. Argillaceous sandstone and shale, red, or mottled with red and gray. 



1. Argillaceous sandstone, graduating below into the Oneida conglomerate. 



In the Genesee section (fig. 357) the strata 1 and 2 correspond to 2 and 3 of 

 these divisions ; and the Niagara section contains 2, 3, and 4. 



Structural peculiarities. — The beds bear evidence of having been 

 formed as a sand-flat or reef-accumulation. Besides the thin 

 lamination alluded to, they abound in ripple-marked slabs (fig. 62) ; 

 mud-cracks (figs. 64,65), due to sun-drying; wave-lines; rill-marks 

 about stones and shells (fig. 63) ; and diagonal lamination (fig. 61 e), 

 an effect of tidal currents. Fig. 63 is drawn from a slab of Medina 

 sandstone. All these peculiarities evince that the accumulations, 

 while forming, were partly in the face of the waves and currents, 

 and partly exposed above the waves to the drying air or sun and 

 to the rills running down a beach on the retreat of the tides or 



II. Life. 



1. Plants. 



Sea-weeds are common, and especially the Fucoid Arthrophycus 

 Harlani (fig. 358). The stems are transversely furrowed, and look a 

 little as if jointed, — to which peculiarity the name alludes (from 

 apdpov, a joint, and Qvtcog, fucus, a kind of leathery sea-weed). 



