January 27, 1922] 



SCIENCE 



87 



imations to the truth; yet they show certain 

 general facts that are fairly well substantiated. 

 The more detailed studies of this kind have 

 made us realize that throughout the recorded 

 part of geologic history there have been alter- 

 nate expansions and contractions of the oceans, 

 with reciprocal changes in the land areas. 

 Sehuehert has estimated that about the middle 

 of the Ordovician time the sea covered two 

 thirds or more of what we now call North 

 America, as well as large parts of other con- 

 tinents. It was one of the distinctively oceanic 

 epochs. On the other hand, the epochs that 

 began and terminated the Paleozoic era were 

 times of diastrophic activity marked by such 

 extreme contractions of the ocean-covered areas 

 that we can find on our present lands only 

 small traces of the marine strata of those ages. 

 The rest of them are doubtless buried beneath 

 the present oceans. 



These oscillations of the shore-line seem to 

 be wholly irregular and pulsating. We can 

 discern no definite tendency toward either a 

 progressive contraction or a progressive expan- 

 sion of the ocean. If there be such a tendency, 

 it must be so gradual that even many geologic 

 periods are not sufficient to reveal its nature to 

 us. Only on theoretical grounds may we sus- 

 pect that the oceans are constantly growing a 

 little larger, for they are receiving from time 

 to time water derived from the steam of vol- 

 canic eruptions; and there is reason to think 

 that much of this water is a real contribution 

 from the interior of the earth and that it has 

 never before been a part of the hydrosphere. 



We have come to understand that the hills, 

 mountains, valleys and other topographic de- 

 tails of the land are almost entirely the result 

 of what James Geikie has aptly called "land 

 sculpture." Streams of water, the wind, gla- 

 ciers, the waves of the sea and still other 

 agencies combine their efforts to produce a 

 composite result, which in some regions may 

 be more the work of one agency and in others 

 largely the work of another. 



On the average these various processes of 

 erosion seem to work with exceeding slowness. 

 The pyramids of Egypt, now nearly six thou- 

 sand years old, have suffered only trivial Avear 

 from the continual sandblast action of the 

 desert winds through all those centuries. From 



estimates of the amount of material carried out 

 by the streams to th6 ocean in solution and in 

 suspension each year, we may calculate that 

 the continents are being worn away at a rate 

 which, if evenly distributed, would be sufficient 

 to lower the land surface one foot in about 

 every 10,000 years ; but we are well aware that 

 this erosion is much more rapid in a few spe- 

 cially favored localities than elsewhere and 

 hence that over most of the land the rate of 

 denudation is correspondingly slower. 



On the other hand, we have clear evidence 

 that these processes have brought down nearly 

 to base-level large areas of land surface in a 

 mere fraction of one of our geologic periods. 

 The Paleozoic rocks of southern Oklahoma, 

 compressed, folded and strongly elevated late 

 in the Pennsylvanian or Carboniferous period, 

 were beveled oft to such an extent before the 

 Permian period that the rocks of that age now 

 rest upon the truncated edges of the older beds, 

 although but a fraction of the Pennsylvanian 

 period intervenes between the two events. In 

 the Rocky Mountains of Colorado and 

 Wyoming an interval of active erosion, follow- 

 ing the first uplift of the Rocky Mountain 

 system, stripjoed off the many thousands of 

 feet of sedimentary formations to such an 

 extent that the old pre- Cambrian crystalline 

 terranes were exposed to erosion before the 

 oldest Tertiary strata were deposited. This 

 lapse of time, although amounting, no doubt, 

 to hundreds of thousands of 3'ears, was so 

 short that it now appears to us only the quick 

 transition between two of our commonly rec- 

 ognized geologic periods. Few things show 

 more clearly how coarse are the divisions on 

 the great time-scale of earth history. 



Our knowledge of the climates of even the 

 better known periods of geologic history has 

 lagged behind that of the jahysiographic and 

 biologic changes. For a long time inferences 

 were drawn almost entirely from the nature 

 of the fossils. Coral reefs were understood to 

 mean tropical or sub-tropical climates. The 

 bones of reindeer in France and of musk-oxen 

 in Kentucky indicated a colder climate than 

 that enjoyed by the same regions to-day. On 

 the other hand, the leaves of pahns and bread- 

 fruit trees, preserved in lignite beds on the 

 coast of Greenland, suggested a much milder 



