GEOLOGIC TIME. 65 I 



mechanical sediments of a uniform character clearly shows this 

 to have been the case, especially in pre-Silurian (Ordovician) 

 time. The present known distribution of the mechanical sedi- 

 ments indicate that they were mainly brought into the sea from 

 the west, 1 although a vast amount was derived from the land 

 on the eastern side in pre-Ordovician time. They were quite 

 evenly distributed over the sea bed, except where local accu- 

 mulations of silt and sand occurred near the larger sources 

 of supply, or in the direction of powerful currents within the 

 sea. 



The conditions of the deposition of the carbonate of lime are 

 less clearly understood than those governing mechanical sedi- 

 ments, and I shall enter upon the discussion of them at consid- 

 erable length. There are three methods by which it usually is 

 considered that it may be deposited: I. Agency of organisms; 

 2. Chemical precipitation ; 3. By mechanical methods. 



It is the general opinion of geologists that limestone rocks 

 are the result almost entirely of the consolidation of lime 

 removed from the sea water through the agency of life, and that 

 they consist of the remains of foraminifera, crinoids, corals, etc., 

 or their fragments, embedded in a more or less crystalline matrix 

 resulting from subsequent alteration of the original deposits. 

 This, however, has been seriously questioned. Sorby, in giving 

 his general conclusions of an extensive microscopic examination 

 of limestones, states that : 



Even if it were possible to study in a detached state the finer 

 granular particles which constitute so large a part of many lime- 

 stone formations, it would usually be impossible to say whether 

 they had been derived from organisms which can decay down 

 into granules, or from other organisms which can only be worn 

 down into granules, or from ground-down older limestone, or, in 

 some cases, from carbonate of lime deposited chemically as gran- 

 ules The shape and character of the identifiable 



fragments do, indeed, prove that much of this must have been 

 derived from the decayed and worn-down calcareous organisms ; 

 r Geol. Expl. Fortieth Parallel, Vol. I., 1878, p. 247. 



