8 Schuchert and Barrett — Revised Geologic 



often leave traces of themselves in the form of teeth, scales, or 

 scattered bones, but their remains are apt to be very fragmentary, 

 due to the streaming action of the rivers ; besides, they are so 

 unchanging in specific form through successive periods as to 

 be of little value in chronology. The animals of the dry land, 

 however, are the best of history markers, because they evolve 

 far more quickly under the most changeable and trying of 

 environments. Their remains unfortunately are rarely en- 

 tombed in the sediments, and as a rule those preserved are the 

 unfortunates that have fallen victims to accidents through 

 drowning, or miring in soft places, especially in times of 

 drought, or have been suffocated through protracted outpour- 

 ings of volcanic ashes. 



The evidence of periodic oceanic spread. — The primary 

 principle of period value underlying geologic chronology is the 

 recognition of the times when the surface of the earth and the 

 oceanic level are in decided motion. This movement may be 

 of small and narrow extent, as the result of horizontal com- 

 pression (local or orogenic) ; or its vertical effects may be felt 

 over areas of great magnitude (epeirogenic). Not only do the 

 lands move up and down, the sum of this motion being in the 

 main upward (positive movements), but it is also now clear 

 that the ocean bottoms are periodically more or less in motion, 

 with the sum of their movements downward (negative move- 

 ments). For these reasons, the oceanic level in relation to the 

 continents is inconstant, and therefore the marine spreading^ 

 over the lands, with their concomitant sedimentation, are not 

 only variable in time, but as well in geographic extent. On 

 the other hand, when the lands protrude more than usual 

 above the strand-line, the oceans naturally overlap the con- 

 tinents least widely and make at such times limited marine 

 stratigraphic records, which are restricted to the margins and 

 their embayments and to the persistent N axes of depression, the 

 geosynclines of the continent. As the oceans and seas are all 

 connected one with another, and are as well the receivers of 

 most of the land wash or detritus, it follows that a displacement 

 of the strand-line anywhere, through any cause, must be trans- 

 mitted to all marine waters. Then under these waters there is 

 continuous sedimentation, and they abound in more or less of 

 evolving life that is most advantageously situated for burial 

 and preservation ; hence the marine stratigraphic sequence is 

 the least broken of the several kinds of historic records acces- 

 sible to geologists. 



It is now known that the oceans have spread periodically 

 and more or less widely over the North American continent. 

 the areal extent of which is about 8,300,000 square miles. 

 These floods occurred hardly at all during the Oenozoic, 1 per 



