CONDITIONS OF SEDIMENTARY DEPOSITION. 519 



shells and organisms remarkably free from mud. Now it may 

 be conceived that the layer of mud on which the creatures live, 

 die, and with sunken organic remains decay, grades from the 

 fresh surface of recent accumulations downward into a much 

 more completely decayed and dissolved mass, and that this 

 rests upon a surface of limestone. In the upper part of this 

 unconsolidated stratum carbonic acid may most abundantly be 

 evolved ; in its lowest part the more concentrated solution of 

 lime may accumulate. Then it is conceivable that lithifaction 

 by crystallization of the carbonate of lime from the more con- 

 centrated solution is constantly proceeding on the limestone 

 surface. If this conception be correct the formation of lime- 

 stone by organic means involves the re-solution and crystalliza- 

 tion of more or less of the calcite in the primary formation, and 

 only those organic forms can remain unchanged which resist the 

 solvent action. If they are delicate, as the trilobites' branchia 

 from the Trenton limestones, described by Walcott, they give 

 evidence that they were rapidly buried and protected. 



It is thought by some that limestones are evidences of organic 

 life at whatever period of sedimentary history they were deposited, 

 but it has here been shown that the source of all lime in the 

 sea is the land, and that, under conditions existing in certain 

 localities, both crystalline limestone and calcareous mud are now 

 forming chemically. It has also been shown that lime converted 

 into organic forms is subtracted from that which would other- 

 wise go to saturate the sea-water. If, then, in any early age of 

 the earth's history, lime-using organisms were not present to 

 subtract and deposit lime from sea-water, and if the atmospheric 

 agencies worked then as now, the contributions from the land 

 must have continually added to the alkalinity of the sea until 

 chemical precipitation occurred. Such a process must have been 

 limited to seas rather than extended to oceans, because the con- 

 ditions of delivery of lime from the land were then, as now, 

 localized. With the development of marine life and the increased 

 demand for lime for organic use, and with the corresponding 

 deposition of organic limestone, the sea-water must have become 



