594 TEE POPULAR SCIENCE MONTHLY 



in the general region concerned; while less conclusive as to the con- 

 temporaneity of widely separated though analogous faunas." 1 



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 geo- 

 graphic position of sandstones and especially on conglomerates to indi- 

 cate 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 evi- 

 dence 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 tem- 

 perature 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 tem- 

 perature 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 sub- 

 merged 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 dis- 

 conformities of stratigraphers. These breaks are at times as extensive 

 as the North American continent (post Utica break), and are usually 



1 Dall, Jour. Geol, 1909, 494. 



