392 PALAEOZOIC TIME. 



mont rock at Sudbury and elsewhere has afforded a few fossils, and 

 these favor its belonging to the Trenton period, as long since held 

 by Rogers. Among them there is a Cyathophylloid coral ; and none 

 of similar character is known before the Pelraia of the Tren- 

 ton limestone; also species referred to Chcetetes, Stromatopora (or 

 Stromatoccrivm) , Stictopora, and Euomphalus. In its great thickness the 

 limestone differs vastly from that of the Calciferous beds of the 

 Quebec group, and has its only counterpart in the Appalachian 

 region in the limestones of the Trenton period in Pennsylvania, 

 which are nearly 6000 feet thick. Admitting this age for the lime- 

 stones, the Green Mountain region did not become dry land until 

 the close of either the Trenton or Hudson period. 



Besides an emergence of the western part of the Green Mountain 

 region at the close of the Lower Silurian, there probably also 

 occurred the metamorphism of its rocks, making the shales and 

 earthy limestones into hard slates and crystalline limestones, to be 

 afterwards overlaid unconformably by later beds. It was probably 

 at this time that the Stockbridge Eolian limestone became a statuary 

 marble and its fossils were almost entirely obliterated. In the 

 same operation, the rocky crust may have received that stiffening 

 which made it a part of the permanent dry land, like the adjoining 

 Azoic, and in consequence of which a district over which a sub- 

 sidence of many thousands of feet took place in the Lower Silurian 

 (p. 176) participated but little in the later Appalachian move- 

 ments. 



The Ottawa basin and the subsiding area of Keweenaw Point on 

 Lake Superior also became comparatively stable before the Upper 

 Silurian era began (pp. 199, 228). 



In a few places there were dislocations of the strata after the close 

 of the Lower Silurian, and again others before the Coal period. 

 These are mentioned on pp. 227, 320. But in general the strata 

 from the bottom of the Silurian to the top of the Carboniferous 

 make an unbroken series, with no unconformability except the 

 slight want of parallelism the great oscillations at times occasioned 

 (p. 320). The great extent of the series, and the vast length of time 

 occupied by those passing ages, make this exemption from great 

 disturbances a subject of profound importance in American geo- 

 logical history. 



4. Direction of oscillations. — The direction of the great oscillations 

 of the continent may be learned from the course of the region 

 along which, through the successive periods, the greatest amount 

 of change of level took place. One such region is the Appalachian, 

 in which the subsidence, as has been shown, amounted in some 



