21 G ANNUAL OF SCIENTIFIC DISCOVERY. 



by the evaporation of calcareous water clashed upon the dry sur- 

 faces. 



" In very shallow waters which are not powerfully affected by 

 tidal movements, and upon the bottom of which no oolites are 

 forming, we lind extensive beds of a dull amorphous limestone, 

 formed of lime mud, alternating with seams of a more compact 

 hard limestone in which a few oolites may occasionally be seen 

 that were floated over the flats in which such formations are 

 going on. These deposits resemble the marly limestone of the 

 Oxford beds. Of course these different rocks may alternate with 

 one another as, owing to the increase of the whole formation, the 

 conditions for the deposition of one kind of rock may be followed 

 by those favoring another combination. Again, in consequence 

 of the changes in the directions of the currents, or as the result 

 of a heavy gale, considerable deposits, which have been going on 

 regularly for a long time, may suddenly be worn away and 

 destroyed, giving rise in turn to the formation of conglomerates 

 made up of limestone fragments of various structure united 

 'together into very peculiar conglomeratic pudding-stone with 

 angular materials. The compact limestones are frequently as 

 hard as the hardest limestones of the secondary formation, have 

 a conchoidal fracture, like the most compact Muschelkalk of the 

 triassic period, and may ring under the hammer. 



"The extensive area occupied by the keys and reefs of Florida, 

 including the sloping coral plateau of the American side of the Gulf 

 Stream bottom, may fairly be compared to the Jurassic formation 

 as it stretches across Central Europe and further east in the 

 direction of the Caucasian and Himalaya Mountains. Indeed, 

 the, Jurassic formation, as a whole, bears the same relation to the 

 older deposits upon which it rests, as the modern American coral 

 formation sustains to the older parts of the coast of our continent. 

 During the geological middle ages the Jurassic formation was the 

 submarine margin of a growing continent, as the Pourtales plateau 

 forms at present the southern margin of North America." 



DISTRIBUTION OF MARINE LIFE. 



*' The Liylitning and Porcupine dredgings have fully established 

 the position that the distribution of marine life is much more 

 closely related to the temperature of the ocean-bottom than to its 

 depth. This is most clearly evidenced by the results of the careful 

 exploration of the channel of from 500 to 650 fathoms' depth, 

 which separates the plateau which supports the northern extrem- 

 ity of Scotland from the Faroe Banks. For we have shown that 

 while the surface temperature of the channels is everywhere nearly 

 the same, and indicates the derivation of its upper stratum from 

 a warmer source, a considerable part of the deeper portion of this 

 channel is covered by a frigid stream, bringing a temperature as 

 low as 29.5 from the Arctic Ocean ; this stream having in some 

 places a depth of 2,000 feet. Thus the bottom temperature, at depths 

 of from 100 to 600 fathoms, is about 45, while there is a cold area, 



