222 GEOLOGY. 



most of the great area between the Appalachian mountains on the 

 east and the Rocky mountains on the west was above the sea during 

 the early part of the Cambrian period. 



Geography of the Early Cambrian. — On the basis of the known dis- 

 tribution of the Lower Cambrian strata as cited above, it is inferred 

 that there was an extended land area within the limits of North 

 America in the early part of the Cambrian period, and from the wide- 

 spread unconformity of rthe Lower Cambrian on its bed, it is inferred 

 that this area was still more extensive just before the deposition of the 

 known Lower Cambrian. The map (Fig. 90) shows, as nearly as it 

 is now known, thb'»area of land and water within the area of North 

 America during the ^arly part of the period. 



" Lost " interval. — Where the Cambrian is unconformable on pre- 

 Cambrian formations there is a break in the geological record. Such 

 breaks are sometimes ;said«to represent "lost" intervals, the intervals 

 which are "lost" beirigi the periods elapsing between the deposition of 

 the beds below the a unconformity, and those above. This designa- 

 tion for such an interval is only partly true, for in the unconforrnity 

 itself there is the record of exposure and erosion, followed by submerg- 

 ence and deposition. The general sequence of events is evident, and 

 to this extent the record is not lost. On the other hand, the products 

 of the recorded erosion were deposited elsewhere, constituting a new 

 formation. Wherever the sedimentary beds deposited during the 

 erosion interval represented by an unconformity are buried, the 

 record of the interval is certainly not open to inspection. It may be 

 regarded as "lost," or as too well preserved. 



The relations and principles involved are illustrated by Figs. 91 to 93. Let 

 the line A A', Fig. 91, represent the sea-level at the beginning of the uplift which 

 closed the Proterozoic era, while CC, Fig. 92, represents the same level at the 

 time of maximum post-Proterozoic uplift. While the water-level was changing 

 from AA' to CC', sedimentation was in progress beneath the water at all stages, 

 and at all stages the sedimentary beds were essentially conformable on the for- 

 mations below. So long as the sea-level stood at CC, deposition still continued 

 to the right of C, the beds being conformable on their substratum (1, Fig. 92). 



Later, as the stratigraphic relations of the Lower Cambrian formations show, 

 the sea again advanced on the land, and as the shore line was shifted from C 

 toward A (Fig. 92) , sediments were in process of deposition on the surface which 

 had recently been land, but which was now submerged. The deposits made at 

 this stage between C and A, Fig. 93, were unconformable on the formations 

 beneath, though the corresponding deposits to the right of C were conformable 



