478 GEOLOGY 



The great tract between the Appalachian Mountains on the one 

 hand, and western Montana and Utah on the other is believed to 

 have been land during the early part of the period. From this 

 land, sediments were probably being carried to the sea on either 

 hand. These sediments, subsequently cemented into rocks, made 

 some of the shales and sandstones of the Lower Cambrian. 



The unconformity between the Lower Cambrian and its base 

 represents what is sometimes called a "lost" interval. This desig- 

 nation for such an interval is not altogether appropriate, for the 

 unconformity records a time of exposure and erosion, followed by 

 submergence and deposition. 



The Middle Cambrian. The strata of the Middle (Acadian) 

 series of the Cambrian system are found with those of the Lower 

 Cambrian, and in addition they are known in Texas, Oklahoma, 

 Arizona, some parts of Montana, and perhaps elsewhere. Since 

 the Middle Cambrian beds contain marine fossils, their distribu- 

 tion indicates that the continent was being invaded by the sea 

 from the south before the close of the Middle Cambrian epoch. 

 Like the preceding series, the Middle Cambrian beds are absent 

 from much of the interior, if present identifications are correct. 

 Where the Middle Cambrian rests on the Lower, the two are 

 generally conformable. 1 Where the Middle overlaps the Lower, 

 it is unconformable on older formations. 



The Upper Cambrian. In the Later Cambrian (Potsdam or 

 Saratogan) epoch, the sea covered much more of the continent, for 

 the Potsdam series covers not only the eastern and western borders 

 of the continent, but much of the interior as well. The Upper 

 Cambrian is conformable on the Middle Cambrian in the east and 

 west, but in the interior it rests unconformably on pre-Cambrian 

 formations. Fig. 358 shows something of the distribution of 

 the Cambrian system as a whole (see explanation beneath the 



Fig.) 



Great submergence during the Cambrian. The distribution of 



the several series of the system shows that the great physical event of 



1 Ulrich and Schuchert think that the Appalachian synclinorium of Marly 

 Cambrian times was largely drained at the close of that epoch. Hull. ">_' 

 (Paleontology 6) N. Y. Mus. Kept, of the State Paleontologist, 1901, p. (>:;<>. 



