828 J. BAKRELL MEASUREMENTS OF GEOLOGIC TIME 



of the tides, of day and night, of the year, of the precession of the 

 equinoxes, and of eccentricity of the earth's orbit. 



Of these, the tides act as geological agents on embayed shores and in 

 straits, but are too rapid in occurrence to leave, except rarely, a record of 

 individual ebb and flow. 



The alternation of day and night serves as an important geological 

 agent in producing a rapid and marked rhythmic recurrence of tempera- 

 ture changes. 



The temperature rhythm of the seasons is now felt in all but a few 

 parts of the earth, and outside of the tropics enters strikingly into the 

 nature of geological processes. Even the year, however, is too brief a 

 space of time to find stratigraphic record exicept in special cases, the 

 most notable perhaps being in the annual rhythm of clay and silt laid 

 down on the bottoms of lakes, especially those which face glaciers and 

 receive during the melting season an abundant supply of waste. One of 

 the most striking facts of geologic history is, however, the absence of 

 freezing cold through most of geologic times in middle and high latitudes. 

 Celestial mechanics points with certainty to a fixity of the poles and a 

 constancy in their inclination to the orbit of the earth. There must 

 always, therefore, have been seasons of darkness in the polar zones and 

 of diminished solar heat in middle latitrCdes. But in spite of this the 

 testimony of fossils points to the general absence of severe cold or frost. 

 At times of glaciation, however, evidence is found of marked seasonal 

 changes in such latitudes, as in the banded argillites of Permo-Carbon- 

 iferous age found by Sayles near Boston, Massachusetts, and in the Pre- 

 cambrian banded argillites found in Ontario and in Sweden. It seems 

 that in periods of glacial climates there is an emergence of winter. 

 Winter, furthermore, must always have been present in continental in- 

 teriors of higher latitudes as a season of more or less cold, especially 

 whenever the climate was semi-arid or arid. The fur of mammals and 

 the feathers of birds probably find their development in a more prevail- 

 ing existence of seasonal change over the continents of the Mesozoic era 

 than has been suspected from the biologic record of marine waters and 

 of coal swamps. In sediments of fairly rapid accumulation the record 

 of the seasons may have been somewhat more widely kept than is sus- 

 pected. In all semi-arid continental deposits the recurrence of waters 

 has probably had a seasonal basis. Generally, however, it is clear that 

 the annual rhythm does not enter into the stratigraphic record, botli 

 because it is too rapid and has been too generally submerged. Even in 

 the deposits of semi-arid fiuviatile basins the strata are the result of far 

 longer and deeper rhythms. 



