of very wide extent. Scour of the bottom by the currents of the ancient continental seas will not explain away the presence of these truly land times, but it is to be sought in the oscillatory nature of the seas of all time which is probably caused by the periodic unrest of the earth's crust due to earth shrinkage. We agree with Suess that "Every grain of sand which sinks to the bottom of the sea expels, to however trifling a degree, the ocean from its bed," and every movement of the sea-bottoms and the periodic down fracturing of the horsts causes the strand lines to tremble in and out, be they of a positive or transgressive or of a negative or land-making character.
The ancient marine life had similar zoogeographic arrangement to that of the present. It can be grouped into local faunas and these combined into subprovinces, provinces and realms. Their distribution is governed primarily by the presence or absence of land barriers, and secondarily by temperature and latitude. In the present seas temperature is one of the main factors controlling the distribution of the species, but during the geologic ages the climate was, as a rule, far more uniform than now, as we are living under the influence of polar ice caps and a passing glacial period, or possibly even an Interglacial period.
The faunas with which the stratigraphic paleontologist works appear in many instances as suddenly introduced biotas. Our collaborators of half a century ago explained them as Special Creations, but since their time we have learned that the suddenly appearing faunas are not such in reality but only seem to appear rather quickly due to the slowness of sedimentary accumulation. Ulrich estimates that the American Paleozoic has less than 100 mapable units or formations, each with a duration of probably not less than 175,000 years. Accordingly, each foot of average sedimentary rock has taken not less than 833 years to accumulate. Our knowledge regarding the average rate of sedimentary marine accumulation is, however, as yet very insecure, and to make this clear some of the remarks made by Sollas, President of the Geological Society of London (1910), will be quoted. He was led to make these remarks after the reading of a paper by Buckman correlating the Jurassic sections of South Dorset. He said, "The correlation of thin seams with thick deposits was a matter of great importance. . . . It might afford some hints as to the order of magnitude of the scale of time. If we assumed that one foot of sediment might accumulate in a century, in an area of maximum deposition, then in the case of the seam two inches thick, which was represented by 250 feet in the Cotteswolds, the rate of formation would be less probably than 1 foot in 150,000 years." What Ulrich's estimate of time necessary for the accumulation of one foot of average sediment means to migratory faunas may be illustrated by the spreading of Littorina littorea. In the last
