TRANSACTIONS OF THE SECTIONS, 179 



range. Ascending the Mohawk valley, the undulation in the Cambrian rocks disap- 

 pears, and both series become approximately horizontal and parallel, but still with 

 omission of formations. 



South-westward from the Hudson, following the north-west margin of the great 

 Appalachian valley, one may trace this plane of discontinuity as far as Eastern 

 Tennessee, or even into Alabama ; for throughout this whole distance of 800 or 900 

 miles, though there is no lapse of a formation at the plane of contact, or any physical 

 unconformity, there is universally so abrupt and crisp an horizon dividing the 

 strata, in respect to composition, conditions of bedding, and organic remains, and 

 such plain evidence that the upper rock was formed from the wreck of the lower 

 ones, that the conviction is inevitable, that a crust-movement revolutionizing the 

 physical geography extended throughout this whole space. The Levant rocks, though 

 next in succession to the Matinal, and reposing conformably upon them, give evidence 

 of such a movement in every feature of their composition. The lower bed is usually 

 a conglomerate composed of fragments of all the underlying formations of the earlier 

 palseozoic or Cambrian series. Some of its pebbles belong to the Primal sandstone ; 

 some are of chert from the Auroral limestone, and much of the grey sandy matter 

 has evidently come from the Matinal slate group. 



Turning attention to the pheenomena connected with this horizon in other parts of 

 the broad Appalachian basin west of the mountains, it can be shown, that, over half 

 the width of the continent, there exists, notwithstanding an almost absolute horizon- 

 tality and parallelism of the two sets of strata, or the lower and middle palaeozoic 

 series, a true discontinuity in the sequence of the formations. In New York there 

 is a conformable interrupted sequence from the Hudson to Oneida county ; from 

 Oneida to Lake Ontario the Levant conglomerate, or Lowest Silurian stratum, enters 

 the gap and makes the sequence complete. 



But this state of things nowhere again prevails from Lake Ontario westward to 

 Illinois and the Missouri River, nor southward from the Laurentian Lakes to the 

 southern outcrops of the two systems on the borders of Alabama, Arkansas, and 

 Texas. 



The Medina sandstone, a higher Levant stratum, partially fills the break across 

 New York, and across Canada to the Manitoulin islands of Lake Huron, where, 

 after constantly thinning, it dies out. Thence to the western boundary of Iowa the 

 hiatus remains unsupplied by any equivalent throughout this whole distance. Tracing 

 the Surgent or Clinton group, the second Silurian formation ascending along the 

 same plane of discontinuity, it is found, after entering the brake or gap near Schoharie, 

 to stretch westward to the Niagara River, and north-westward to the Manitoulins, 

 and possibly thence to Green Bay. Beyond the Niagara River it is an extremely 

 thin bed of limestone and calcareous shale. Thus from the peninsula of Michigan to 

 the cretaceous plains of the Missouri, two entire formations are omitted above the 

 top of the lower palaeozoic or Cambrian formations. The Sealant or Niagara series, 

 the third Silurian group, ranges through a wider zone. Thin and obscure in the 

 eastern part of New York, and almost gone in the Appalachian chain from the 

 Hudson southward, it is an important stratum from western New York westward 

 to its disappearance beneath its cretaceous covering in the plains of Nebraska. It 

 was the first middle palaeozoic or Silurian deposit, formed upon the floor of the old 

 Appalachian sea, upon its re-immersion after its upheaval at the close of the Matinal 

 period. 



Reviewing these statements, we arrive at this interesting general picture : — 1st, a 

 violent and universal agitation of the whole bed of the Appalachian palaeozoic ocean 

 at the close of the Matinal period, resulting in its upheaval and drainage, from the 

 region of the Gulf of St. Lawrence to that of the centre of the continent, and in a 

 general shoaling of every other portion. 2nd, a more local paroxysmal movement 

 of depression accompanied by the formation of the Levant or Oneida conglomerate, 

 followed by a gradual and successive subsidence, letting in the ocean over a wider 

 space during the Levant and Surgent periods, until in the Scalent or Niagara period 

 the whole area was reclaimed again by the ocean. In the first stage of the subsidence, 

 the sea filled only a long, narrow trough, parallel with the present Appalachians ; in 

 the next or Median age, it had spread along its northern coast westward as far as 

 Lake Huron, but was evidently very shallow ; and in the following or Surgent period, 



12* 



