48 THE MODE OP OCCUEEENCE OF EOCKS 



begins to deposit its load, the coarsest and heaviest first, and 

 the finer materials further from the shore, the very finest, an 

 impalpable silt it may be, remaining suspended until the very 

 last. There will thus be formed a bed, or series of beds of vary- 

 ing thickness, of gravel, sand, and clay, the coarsest at the 

 bottom and nearest the shore, and the finest and last the most 

 remote. 



But the streams vary from time to time in their carrying 

 capacity, and the action of the waves and tides together with the 

 dissolved salts, exert a modifying action, whereby this process 

 of sedimentation^ as it is called, may not be quite so simple as it 

 first appears.^ Enough has, however, been said to show that 

 beds of detritus laid down in this manner must occur in ap- 

 proximately horizontal layers, and that the layers may vary 

 greatly in the coarseness and fineness of their materials, as well 

 as in their mineral character. But there are still other processes 

 of sedimentation than the purely mechanical methods described 

 above. All natural waters contain more or less mineral matter, 

 of which lime is the more abundant. Through the secreting 

 power of marine animals, this lime is taken up in the form of 

 a carbonate to form shells and calcareous skeletons of molluscs, 

 corals, and other forms of marine life. On the death of the 

 secreting animal, the calcareous material is left to accumulate 

 in a more or less fragmental condition, forming thus the material 

 of the coral islands, and to a considerable extent the beds of lime- 

 stone the world over. The expression, to a considerable extent, 

 is used for the reason that it is doubtful if all of our limestones 

 are of purely animal origin ; in many a true chemical precipita- 

 tion plays a not unimportant part. This is especially true of 

 the oolitic varieties, and the fact is readily apparent when one 

 studies such in detail. Consider a shallow sea-bottom on which 

 are gradually accumulating in a finely divided condition the 

 fragmental remains of calcareous organisms of any kind. By 

 the undulatory action of the waves these are kept in almost 

 constant motion, though it may be but gently rolling from side 

 to side. Owing to evaporation, or a too rapid accumulation of 

 the lime for it to be abstracted by the lime-secreting animals, 

 the water becomes supercharged with this constituent, which is 

 then precipitated in the form of a thin pellicle around the most 



^ See Conditions of Sedimentary Deposition, by Bailey Willis, Journal of 

 Geology, 1893, p. 476. 



