Time-table f vr Worth America. 9 



cent to 6 per cent of the continent being then covered ; four 

 times widely during the Mesozoic, the submergences reaching 

 3 per cent to 33 per cent ; and, with the maximum spread, 

 apparently eleven times during the Paleozoic, when 1 per cent 

 to 47 per cent of the continent was flooded. More broadly it 

 may be stated that the floods begin and end with shelf seas 

 marginal to the continents and varying in extent between 1 per 

 cent and 5 per cent of the total areas of the continental platform, 

 the conditions being thus not unlike the present conditions of 

 overlap ; while the greatest inundations during the middle of 

 the periods attain from 12 per cent to 47 per cent of the con- 

 tinent. 



It is therefore apparent why the major portion of the earth's 

 chronology depends for its determination upon the marine 

 sediments. These formations, except in so far as they are later 

 eroded, record the extent of the transgressions, and, in their 

 physical characters, something of the topographic form of the 

 adjacent lands, with a hint as well of their climates ; and 

 through their fossils they establish the chronology from place 

 to place. However, this is by no means all, for the newer 

 geology also teaches, as we have seen, that the strand-line is 

 constantly and geographically irregular in motion, either very 

 slowly transgressing more or less of this or that land, or reced- 

 ing as the lands emerge. Therefore in no land is there a total 

 record, but everywhere the story is more or less incomplete, 

 and our chronology is but a patchwork of all the local histories 

 pieced together into one still very imperfect geologic time- 

 table. 



In the marine formations we are then everywhere dealing 

 with oceanic overlaps whose records for the time being were 

 more or less complete, but each series of beds is nearly every- 

 where separated from the adjacent ones by erosion intervals. 

 The latter are due to the periodically recurring emergent times 

 in the history of the continents, which may be either local or 

 of wide extent, the marine records in the latter case being 

 swept away by the atmospheric forces. These erosion intervals 

 are the " breaks " and they are not only significant of absence 

 of sedimentation, but in addition are actually records of another 

 type, that is, erosion histories resulting in topographic forms 

 whose carving has required the lapse of a time more or less 

 long. 



Just as the marine waters are constantly registering their 

 existence, so also do the fresh waters, but the areas of the latter 

 are usually of comparatively small extent. The " continental 

 deposits" tend to be an evanescent record made at one time 

 only to be subsequently more or less completely swept away 

 into the sea. When we observe that this record is made in the 



