Page:Popular Science Monthly Volume 76.djvu/598

594 in the general region concerned; while less conclusive as to the contemporaneity of widely separated though analogous faunas."

In regard to the probable geographic position of the shore lines we rarely have safe guidance in the fossils, and for this depend on the nature of the deposits. Greatest dependence is placed upon the geographic position of sandstones and especially on conglomerates to indicate the probable former shores. Limestones of uniform character and wide distribution are indicative of greater distance from land. Shallowness of the continental seas is proved by a rapid change in the character of the sediments both laterally and vertically, and by the oolite and dolomite deposits. Intraformational conglomerates, coral reefs, ripple marks, and shrinkage cracking furnish further evidence to the same conclusion. Storm waves are known to plough the present sea-bottom to depths of 160 feet. Calcareous muds are now forming in tropical and subtropical waters at sea-level around coral reefs, and elsewhere in these latitudes at depths from 200 to 600 meters. It is probable that all of the ancient great limestone deposits are of warm waters, and, if so, are an additional aid in discerning the geologic times and regions of milder climates.

Phosphatic concretions form in the littoral region where the temperature changes are rapid, as off the coast of the New England states, and periodically cause much destruction of the individual life. The carcasses decompose at the bottom of the sea, making nuclei for the accretion of phosphate of lime, and because of the irregular periodicity of accumulation come to be arranged in definite stratigraphic zones. Old Red sandstone fishes are also usually found in clay nodules but abundantly only in limited zones (Scaumenac, Canada and Wildungen, Germany). Have these also been killed by rapid changes in the temperature of these waters? In any event the fish-bearing beds are always found near the shore lines of Devonic seas.

Scour of sea bottom is met with in the present seas where great streams of water are forced through narrow passages, as the Gulf Stream in the Floridian area; or where such streams impinge against the continental shelf, as north of Cape Hatteras, or flow across submerged barriers "a few miles broad," as the Wyville-Thomson ridge connecting the British and Faeroese plateaus (Johnstone, 1908, 31). Strong currents preventing sedimentation also occur in long and narrow bays, as that of Fundy, where the undertow caused by the very high tides of this region sweeps the bottom clean. These exceptional and, after all is said, rather local occurrences can not be the explanation for the many known breaks in the geological sedimentary record, the disconformities of stratigraphers. These breaks are at times as extensive as the North American continent (post Utica break), and are usually