GEOLOGIC TIME. 319 



sented; altliough they are now living in immense nnmbers in the sur- 

 face waters or on the deposits at the bottom in some regions, yet all 

 traces of them have been removed by solution. A similar removal of 

 calcareous organic structures has undoubtedly taken place in the 

 marine formations of past geological ages."* 



From the preceding statements it is evident that initially the greater 

 part of the carbonate of lime is taken from the sea water by organic 

 agency, but in the working over of this material in the chemical labo- 

 ratory at the bottom of the sea a considerable portion is taken up by 

 the sea water as amorphous carbonate of lime and thrown out in the 

 crystalline form to form the matrix of the undissolved shells, etc. t 



Mr. Bailey Willis has recently studied the question of the deposi- 

 tion of carbonate of lime, and states that "chemists describe two con- 

 ditions under which bicarbonate of lime may be decomposed into 

 neutral carbonate and carbonic acid: first, by diminution of the ten- 

 sion of the carbonic acid in the atmosphere; second, by agitation of the 

 solution, 



"Theoretically either one of three things may occur to the neutral 

 carbonate of lime, if it be thrown out of solution ])y either one of these 

 processes. The carbonate may be redissolved, deposited as a calca- 

 reous mud, or built into organic structures." He studied some recent 

 limestone deposited in the Everglades of southern Florida and found it 

 to be formed of fragments of shells embedded in caicite. He states 

 that " under the microscope the unaltered structure of the organic frag- 

 ments is strikingly different from that of the coarse holocrystalline 

 matrix, in whi(;h it is apparent that the crystals developed in place. 

 Were this a limestone of some past geologic period it would be con- 

 cluded, on the evidence of the crystalline texture of some parts of it, 

 that it had been metamorphosed and that tlie organic remains now 

 visible had escaped the process which altered the matrix. But the 

 observed conditions of its fornnition preclude the hypothesis of sec- 

 ondary crystallization."! Apparently the crystalline matrix is one 

 ]nimary product, and the calcareous mud is another, which being pre- 

 cipitated in the solution remains an incoherent sediment. 



I think we may accept the conclusion that the deposition of carbonate 

 of lime is by both organic agency and chemical precipitaticni. It is not 

 necessary to speak of deposition by mechanical methods except in rela- 

 tion to the dej)osition of chemically derived granules. This probably 

 takes place, and may be a very important factor in the formation of 

 limestones in seas receiving a large supply of calcium from the land. 

 Calcareous conglomerates do not enter as a prominent deposit in the 

 Cordilleran area. 



* Loc. cit., p, 277. In this connection I wish to ask the student to read Messrs. 

 Murray and Irvine's remarks on pp. 97-99, Proc. Boyal Soc. Edinhnrcih, 1890, vol. xvil. 

 f Proc. Royal Soc. Edinburgh, 1890, vol. xvii, pp. 94, 95. 

 X See Mr. Willis's article in Journal of Geology, Chicago, July-August, 1893. 



