328 HISTORICAL GEOLOGY 



be deposited. This unconformity evidently tells of a very 

 long lapse of time between the deposition of the Archaean 

 and Proterozoic rocks, a time otherwise unrecorded in the 

 rocks which we know. Because of this, and because of the 

 wide distribution of the unconformity, it is generally regarded 

 as one of the greatest interruptions in the geologic record. 

 But no unconformity, however widespread, can exist all over 

 the globe. The very same facts which indicate that the lands 

 were deeply eroded prove that the material worn off was as 

 continually being deposited elsewhere; and in those areas 

 where deposition was in progress no unconformity resulted. 

 It has been suggested that the sediments which were deposited 

 then, as now, in the deep ocean basins have never been raised 

 into land, and hence are still unknown to us. 



Unconformities within the Proterozoic group. Other 

 notable unconformities serve to divide the Proterozoic into 

 two or more systems. In Minnesota the Animikean sand- 

 stones and shales rest at a moderate inclination upon closely 

 folded slates and quartzites of the Huronian. Furthermore, 

 some of the dikes in the Huronian rocks do not pass up into 

 the Animikean system. Such an unconformity is conspicuous 

 when it can be seen in the side of a quarry or a ravine. No 

 one of these interruptions in the strata has, however, been 

 traced across any continent, much less over several conti- 

 nents ; and the divisions themselves, therefore, can be used 

 only in the region where they are known to apply. Protero- 

 zoic rocks are generally separated from the Cambrian system, 

 which overlies them, by another great unconformity, a de- 

 scription of which will be found in the next Chapter. 



Duration of the Proterozoic era. Just as the remote 

 ancient periods of human history are long in comparison with 

 the subsequent centuries, so the Proterozoic era was im- 

 mensely long as compared with later periods. In the course of 

 a century only a few feet of average sediments are deposited, 

 and of limestones perhaps not even one foot. Yet the Huro- 

 nian sediments of Michigan are alone said to be more than 



