282 PALAEOZOIC TIME — DEVONIAN AGE. 



The Marcellus shale rarely exceeds in thickness 50 feet. The Hamilton strata 

 are 1000 feet thick in central New York, but not half this along Lake Erie. 

 They are also comparatively thin and more sandy on the east in the Helderberg 

 Mountains. They are well exposed along the valleys of Seneca and Cayuga 

 Lakes. The Genesee shale is 150 feet thick near Seneca Lake; it thins west- 

 ward, and is not over 25 feet on Lake Erie. 



Still farther west it is represented by what is called the Black slate, — rather a 

 shale, — a very persistent stratum occurring in Tennessee, Kentucky, Indiana, 

 and elsewhere; it is a hundred feet thick at Louisville, Ky., and in Indiana. 

 In Missouri, the Hamilton formation consists of about 50 feet of shale, with 

 some beds of limestone. 



b. Appalachian region. — In Pennsylvania, H. D. Rogers makes three divisions 

 of the Hamilton formation, a lower of black shales, which is 250 feet thick in 

 Huntingdon, a middle of variegated shales and flags, 600 feet thick at the same 

 place, and an upper black shale of 300 feet. 



The thickness of the Hamilton formation east of central New York shows 

 that this region was at this time, as in the Oriskany period, on the northern 

 border or limits of the Southern Appalachian region. 



At Gaspe, on the Gulf of St. Lawrence, the Devonian sandstones have a thick- 

 ness of several thousand feet. They are regarded as belonging to the Lower 

 rather than Upper Devonian; but the precise age has not been determined. 

 From the fossil plants it seems probable that the Hamilton period is represented 

 among them. 



c. Eastern border. — At Perry, Maine, there are other Devonian strata con- 

 nected with the Canada deposits, and they also may be of the Hamilton series. 

 The fossils thus far found have not served to fix their precise age. They 

 contain some species of fossil plants identical with those of Gaspe. 



Ripple-marks. — The rocks of this formation, especially the Hamil- 

 ton beds, are remarkable for the abundance of ripple-marks on the 

 layers. The flagging-stone is often covered with ripple-marks and 

 wave-lines. The joints intersecting the strata are often of great 

 extent and regularity. They have been referred to on page 100, 

 and a sketch is there given representing a scene on Cayuga Lake. 

 The rock at the place is the Moscow shale. 



II. Life. 



1. Plants. 

 The carbonaceous material of the black Marcellus shale is mostly 

 due to vegetation ; but whether to sea-weeds or land-plants has not 

 been ascertained. In the Hamilton beds the evidence of verdure 

 over the land is no longer doubtful. The remains show that there 

 were trees, and of large size. Figs. 483 and 484 represent the outer 

 surface of two of the species, showing the scars left by the bases 

 of the fallen leaves. Fig. 483 is Lepidodendron primcevum, from, near 



