THE CAMBRIAN PERIOD. 239 



that the stratigraphic relations of a formation with the formations 

 next beneath do not necessarily determine its age. As the sections 

 show, the Lower, the Middle, and the Upper Cambrian strata in turn 

 rest unconformably upon pre-Cambrian rocks. Just as the Upper Cam- 

 brian overlaps the earlier formations of the period, resting uncon- 

 formably on the pre-Cambrian, so any later formation might overlap 

 the Upper Cambrian and sustain similar relations to the Proterozoic 

 and Archean systems. It follows that where one formation is uncon- 

 formable on another of known age, the stratigraphic relations between 

 them do not determine the age of the younger, beyond the general 

 fact that it is younger than the base on which it rests. If, on the other 

 hand, a formation is conformable on another of known age, the pre- 

 sumption is strong that the upper succeeded the lower without inter- 

 ruption. If this inference be correct, the approximate age of the upper 

 is known. But even here there is possibility of error. If, after a 

 formation is deposited, the sea bottom where it lies is brought above 

 water level without deformation of the beds, and if the elevation be 

 so slight as not to permit of much subaerial erosion on the newly emerged 

 surface, it might be submerged at a later time and receive a new series 

 of deposits, the beds of which would be stratigraphically conformable, 

 or approximately so, with those below. It is clear, therefore, that the 

 answer to the question proposed in this paragraph is not always to be 

 found in the stratigraphy of the formations concerned. The complete 

 answer involves as well a consideration of the fossils which the forma- 

 tions contain. 



Fossils. — It has been stated that life existed in the preceding eras. 

 Yet in the Proterozoic formations, fossils are exceedingly rare, though 

 it does not follow that life was sparse during their deposition. When, 

 after the lapse of the long interval between the deposition of the young- 

 est Proterozoic and the oldest Cambrian strata, as known in most parts 

 of North America where detailed geological work has been done, the 

 waters again advanced over the land surface, making the deposits which 

 are now known as Cambrian, they teemed with life. As the sediments 

 were deposited, the shells or such other hard parts as the animals pos- 

 sessed, were buried in the gathering sands and muds, and some of them, 

 or the impressions which they made, remain to this day. The Cambrian 

 is therefore the oldest known system of rocks which contains abundant 

 fossils. 



