74 J. G. Goodchild—The Paste of Limestones. 



that nearly the whole mass, in the great majority of cases, is either 

 directty, or indirectly, due to organic agencies. This view is even 

 maintained in the cases where a microscopic examination of the rock 

 fails to reveal more than a few traces of any structure that can be 

 regarded as organic. 



It is perfectly true that the exclusively-organic structure of the 

 greater part of many limestones' does not admit of any question. 

 No one can doubt that fully 90 per cent, of the matter composing, for 

 example, the shell-limestones of the Jurassic rocks, or the encrinital 

 limestones of the Yoredale Series (which consist of masses of en- 

 crinites) must be of organic origin. But there are other limestones, 

 often associated with these, of whose organic origin at all the 

 evidence is by no means clear. Between these two extreme types 

 every intermediate gradation exists. The fact, indeed, may be 

 demonstrated by a careful study of the constitution of the different 

 beds, or posts, of the same limestone, or even of the different lamina? 

 composing the same bed. One such may consist almost entirely of 

 organisms, fragmentary or otherwise; while those associated with 

 it may exhibit hardly a trace of a fossil of any kind. In the case of 

 certain shales associated with limestones, such for example as those 

 of the Yoredale Kocks, the argillaceous bed may be crowded with 

 organisms, while the calcareous bed immediately below may yield 

 but a few traces of such ; or else, in some cases, may consist of 

 little else than an amorphous paste of carbonate of lime, in which 

 neither microscopic examination, nor any other kind of investiga- 

 tion, reveals more than a few traces of organic structure. 



Those writers who have occupied themselves with the study of 

 limestones seem to have given but little attention to the nature and 

 origin of this amorphous constituent of the rock. Perhaps one 

 reason may be that those who have worked most at limestones have 

 been specially interested in fossils, and have regarded the parts of 

 the rock that did not exhibit traces of organic structure as com- 

 paratively devoid of interest. Even Dr. Sorby, in his well-known 

 address on the origin of limestones, passes over this part of the 

 subject with much less notice than one could have wished it should 

 have received from him. He refers it chiefly to the crumbling down 

 of the aragonite of calcareous organisms that were undergoing 

 decomposition ; l but part of it he refers to the detrition of older 

 limestones, and part of it he regards as a chemical precipitate. 

 Professor Prestwich (Geology, vol. ii. p. 320) suggests that the 

 impalpable amorphous, matter that forms so large a part of the 

 Chalk, for example, ' ; may really be a chemical precipitate thrown 

 down under special and peculiar conditions prevailing at the time." 

 Amongst other explanations is that based upon the fact that the 

 seaward face of coral reefs is always undergoing considerable wear 

 and tear under the action of breakers, and that much of the fine 

 chalky matter resulting from this action is distributed far and wide 



1 On this subject see Vaughan Cornish and Percy F. Kendall, " On the Minerj - 

 logical Constitution of Calcareous Organisms," Geol. Mag. Dec. III. Vol. V. 

 p. 66. 



