THE PALEONTOLOGIC RECORD 15 



of very wide extent. Scour of the bottom by the currents of the an- 

 cient 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 transgres- 

 sive 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 tempera- 

 ture 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 slow- 

 ness 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 Geo- 

 logical 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 Cottes- 

 wolds, 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 



