130 



THREE CRUISES OF THE '' BLAKE. 



paleozoic, mesozoic, and subsequent formations were depos- 



' ""aU the evidence we have seems to show that up to the time 

 of the cretaceous, the sedimentary rocks, in spite of then- enor- 

 mous thickness, were deposited in comparatively shallow seas. 

 We find in them abundance of animal remains, fossils, and even 

 tracks, as well as ripple-marks, indicating the proximity of land 



Yig. 62. — Archsean Map of North America. (Dana. ) 



or of continental islands. Undoubtedly, in earlier geological 

 periods there have been invasions upon as well as withdrav^^ls 

 of the sea from the low continental masses resting upon the 

 continental folds, and these invasions have left probably large 

 shallow inland seas upon these folds. Then, as now there 

 must have been a considerable deposit of finer material upon 

 the outer edge of the continental masses then forming, which 



