323] G. E. Dorsey 125 



down under a depth of water varying from a few feet to 200 

 fathoms, and does not mean deep-sea conditions. 



An argument favoring the warm-water origin of the chalk 

 is the result of the work of GL H. Drew., of the Carnegie 

 Institution, upon the precipitation of calcium carbonate from 

 the sea-water by bacterial agencies. He says (23, p. 136), 

 " Denitrifying bacteria possess the power of precipitating 

 soluble calcium salts in the form of calcium carbonate from 

 sea-water .... bacterial action may have formed an im- 

 portant part in the formation of the chalk and other lime- 

 stones rocks in geologic times." These bacilli, called " Bac- 

 terium calcis " by Drew, grew best on or near the surface in 

 water from 25 to 31.5 degrees C., with an average of 29 de- 

 grees; they will grow very slowly at 15 degrees C.; "but its 

 growth is totally inhibited at 10 degrees C." As such tem- 

 perature can only be attained in tropical and sub-tropical 

 oceans today, it is certain that the seas of the chalk must have 

 been of at least this warmth, if any of the chalk is due to 

 bacterial precipitation, a view now very widely held. 



Cayeux (16, p. 258-9), says, referring to the chalk of the 

 Paris Basin, " La craie du Nord est bien un depot terrigene." 

 A. Geikie agrees that it is a shallow water deposit. De Lap- 

 parent still inclines to the belief that it is a fairly deep-sea 

 accumulation. Schuchert (17, p. 885), says the evidence that 

 chalk is a shallow water deposit is, (1) the kinds of fossils 

 indicate shallow water, (2) the formations are accompanied 

 by sands, (3) in closely adjoining areas equivalent strata 

 contain no chalk. He sums the question up as follows: "It 

 is now held that the chalks are organic accumulations made 

 in the main by the calcareous skeletons of minute pelagic or 

 bottom-living plants and animals, in clear-water epeiric or 

 shelf seas, adjacent to low lands with mild climates/ 5 



The most important feature of this summary is the clear- 

 water condition postulated. Recent work on the Selma Chalk 

 has shown it to be a deposit laid down in water into which no 

 muds or large amounts of sand were carried. The calcium car- 

 bonate, freed of all other sediments, accumulated in a com- 



