248 GEOLOGY. 



of the interior is mainly sandstone, while that of the southern Appa- 

 lachians is shale and limestone. Similar variations are encountered 

 everywhere. They are indeed so prevalent that there is no sufficient 

 reason for assuming that the Upper Cambrian of one region is repre- 

 sented by sandstone, because that is the formation of the corresponding 

 time elsewhere. 



Not only were beds of sediment of different sorts accumulated 

 at the same time in different places, but they were accumulated at very 

 different rates, as suggested by the sections in Fig. 96. It should be 

 borne in mind persistently in the study of all stratified formations, 

 that equal thicknesses of rock do not necessarily accumulate in equal 

 periods of time. The full section of the Middle Cambrian, that is, all 

 that was deposited during the whole of the Middle Cambrian epoch, 

 seems to be present at many points, yet the thickness of the Middle 

 Cambrian strata is far from uniform. 



The Lower and Middle Cambrian sediments of the Appalachian 

 belt appear to have been deposited in a narrow interior sea, or in bays, 

 which lay between the pre-Cambrian rocks just east of the present 

 mountains and the great interior land area to the west (Fig. 90). The 

 beds accumulated in this narrow interior sea are such as to indicate 

 that the water was shallow, in spite of the fact that the deposits are 

 estimated to have a thickness of several thousand feet. This apparent 

 contradiction may be the result of an exaggerated estimate of thickness, 

 or, if the estimated thicknesses are correct, it may be explained by 

 supposing that, as the sediments were laid down in the shallow water, 

 the bed on which they gathered sank about as rapidly as they accumu- 

 lated, so that the whole series, from bottom to top, was deposited in 

 shallow water; or the beds may have been built out from the shore 

 in sloping attitudes, their upper parts being always in shallow water. 

 In the later part of the Cambrian period, the narrow Appalachian sea 

 of the earlier epochs had become a part of the wider sea (Fig. 95) 

 which covered much of North America, but the water remained 

 shallow. 



The Cambrian formations along the North Atlantic coast (New 

 England and north) are such as to indicate that the sediments accumu- 

 lated near shore and in shallow water, that during their accumulation 

 there were frequent oscillations of level, but that, on the whole, the 

 sea gradually gained on the land. The distribution of the formations 



