130 THREE CRUISES OF THE “ BLAKE." 
palwozoic, mesozoic, and subsequent formations were depos- 
ited. 
All the evidence we have seems to show that up to the time 
of the cretaceous, the sedimentary rocks, in spite of their 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 
Fig. 62. — Archean Map of North America. (Dana.) 
or of continental islands. Undoubtedly, in earlier geological 
periods there have been invasions upon as well as withdrawals 
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 
