INTRODUCTION. 79 



mountains and in the western part of the continent, are likewise asso- 

 ciated with newer strata, or are of a very modern age. 



In the comparatively slow accumulation over large areas along the 

 course of the Laurentian and Appalachian mountains, the depression 

 would be slowly accomplished, and, as I suppose, comparatively few 

 extensive rents or fractures would be produced. These would be filled, 

 as we find them in the dykes, with rarely overflows of the same mat- 

 ter. On the contrary, we may readily conceive that where very rapid 

 accumulation has taken place over certain areas of limited extent, the 

 crust below might give way, from the overload, and the whole be 

 plunged into the semi-fluid mass beneath, causing it to overflow. 

 Whether this reasoning be correct or otherwise, I believe that the 

 overflows of trappean matter are always coincident with the rapid 

 accumulation of sedimentary materials. 



In the region of Lake Superior, the sandstone, of the age of the 

 Potsdam sandstone, has accumulated to a degree unparalleled in any 

 other known locality of that rock. In this region there are not only 

 massive accumulations of trappean matter, but outflows which have 

 spread over the strata during their deposition ; the beds of stratified 

 amygdaloid trap alternating with the shale and sandstone, often equal- 

 ling or exceeding the sedimentary matter. 



In the period of the New Red sandstone, we have evidences of very 

 rapid sedimentary accumulations; and accompanying the same are large 

 outflows of trappean matter, both in the form of dykes, interstratified 

 and overflowing masses, such as are well described by Messrs. Jackson 

 and Algeb, and by Mr. Dawson in Nova-Scotia. In the Connecticut 

 and Hudson-river valleys, whatever we may say o^ the age of the sand- 

 stone, we have evidence of its rapid accumulation ; and though extend- 

 ing for a long distance in a line parallel to the Appalation chain, it has 

 nowhere a great width, but always meets the conditions first stated, 

 of a great and rapid accumulation within a narrow space. 



Without having the data, at this time, for a comparison of other 

 similar regions, I believe the law will hold true, that all great outbursts 



