784 J. F.AJiRELL MEASUREMENTS OF GEOLOdIC TIME 



per cent limestone. An average of sections aggregating- LSS.OOO feet, 

 scattered tlirongli Enrasia, gives 19 pet' cent shales, 321/. per c-ent sand- 

 stone, and 18^/2 P<^i^ cent limestone. The proportions which thO}' calcu- 

 late from the average chemical composition of igneous rocks should give 

 82 per cent shale, 12 per cent sandstone, and G ]jer cent limestone.^'' 

 Leith and Mead ascribe this discrepancy to tlie probahilit3^ that many 

 of. the sediments consist of sandstones with undeconlposed or unsorted 

 material, inchiding much, shale, and further that many limestones are 

 shaly.^"'' It appears to Professoi' Sclnicliei't and the A\riter, liowe\'er, that 

 this, although it may be a. partial leason, van not ])e an adequate explana- 

 tion. The chemical analyses of sediments shouhl to a considerable degree 

 include formations of these mixed compositions. An important factor in 

 the explanation is the carrying of luud to tlie slopes of the deeaji !)asins, 

 because nearly always an excess of sediment has been supplied to the 

 shallow seas beyond that wdiich was required to keep their floors at wave- 

 base. The mud deposits of these slopes are almost never exposed to ob- 

 servation. Thus this discrepancy between the observed and calculated 

 proportions of sediments is in accord witli the joi'inciples controlling 

 sedimentation which are here emphasized. 



The excess of limestones observed to occur on the continents, above their 

 ratio to the total theoretical volume of shale, is also in line with the jDreva- 

 lent deposition of limestone under the control of w^ave-base. Calcium 

 carbonate is abstracted from sea-w^ater by organisms almost entirely in 

 the zone of surface waters. Of this the part which is permanently lost 

 to the continents is chiefly due to the accumulation of globigerina ooze 

 on many parts of the ocean basins above 3,000 fathoms in depth. This 

 ooze covers 40,000,000 square miles, 29 per cent of the entire ocean sur- 

 face; but, as JMurray and Eenard have shown, the per cent of carbonate 

 decreases with depth, so that the bulk of the permanent deposit is on the 

 higher bottoms. Tlie quantity of pelagic life i^er unit is greater in the 

 waters of the sh'allow seas than in. the central waters of the oceans. Add 

 to this pelagic life the abundance of the lime-secreting bottom life of the 

 sun-lit floors of the shallow seas, and it is seen why the limestones have 

 been chiefly deposited on the continental platforms. 



Thus, theor}^ is in accord with observation in indicating that through 

 geologic time deposition of marine sediments on the contijiental |)lat- 

 forms has been in general conti'olled l)y ^vave-base, the terresirial deposits 



"• Leith and Mead : Metamorpliic Geology, 1915, p. 60: By a typograpliical cri-or the 

 per cents stated in the ))0ok are in error for the Hmestoncs. The corrected figures given 

 above are by Leitli in resjionse to an inquiry liy the writer. 



-" Loc. cit., p. 68. 



