150 GEOLOGY. 



are as striking as the points of likeness. Both consist chiefly of the 

 shells of minute protozoans, largely foraminlfera; but with them are 

 associated shells of other types, some of which are similar in the two 

 formations, and some dissimilar. The echinoderms, the sponge spicules, 

 and the shells of certain microscopic plants found in the chalk seem 

 to correspond in a general way with those of the oozes now depositing, 

 and are consistent with the deep-water origin of the chalk. The mol- 

 luscan shells of the chalk, on the other hand, seem to point with clear- 

 ness to water no more than 30 to 50 fathoms deep. The distribution 

 of the chalk and its relations to other sedimentary beds, seem to point 

 to its deposition in water of moderate depth, rather than in water 

 comparable in depth to that in which oozes are now formed. 1 That 

 chalk may originate in shallow water seems to be clearly indicated by 

 various facts which have been observed in connection with coral reefs, 

 past and present. 2 



Another point of difference between chalk and foraminiferal ooze 

 is found in their relative proportions of CaC0 3 , the proportion being 

 much higher in chalk than in ooze. The elevation and exposure of 

 the chalk can hardly have led to this difference, for the extraction 

 of the relatively soluble lime carbonate must have increased the per- 

 centage of the relatively insoluble impurities. On the other hand, 

 the analyses of chalk which have been used in this comparison may 

 have been from the purer portions of the formation, and since chalk 

 grades off into chalky clay and chalky sandstone, varieties of chalk 

 containing no more lime carbonate than the oozes, are doubtless to 

 be found in abundance. 



One of the peculiarities of the chalk beds is the presence in them 

 of abundant nodules of flint and chert which are not present in the 

 modern deposits resembling the chalk. These nodules seem to have 

 resulted from the subsequent concentration into concretions of the 

 siliceous material (sponge spicules, etc.), deposited along with the 

 calcareous shells which make up the body of the chalk. On the whole, 

 the balance of evidence seems to favor the hypothesis that the known 

 chalk deposits were made in relatively shallow water. The conditions 

 for the origin of the chalk seem to have been clear seas, with a genial 



1 Wallace, Island Life, pp. 89-96. The argument for the shallow water origin 

 of chalk is here forcibly presented. 



2 Dana, U. S. Exploring Expedition. 



