1871.] A Geological Problem. 329 



formed of the same materials — carbonate of lime and animal 

 matter. If, therefore, the argument of similarity of chemical 

 composition is to be trusted, it follows that a rock rich in fossil 

 oysters but poor in fossil cockles must have been formed 

 at a time when, or in a locality where, the former abounded, 

 but the latter were scarce. In 1862, however, Mr. Sorby 

 brought the subject under the notice of the British Associa- 

 tion, and pointed out that the carbonate of lime in the shells 

 of limpets, oysters, and some other mollusks, took the form or 

 condition of calcite, but in cockle shells and their allies, the 

 form of arragonite ; the difference being one, not of chemical 

 composition but of physical arrangement, whereby the 

 molecules were in stable equilibrium in the former, but un- 

 stable in the latter, thus giving the arragonite shells a lia- 

 bility to disappear from which those of calcite were exempt.* 



It is obvious, therefore, that as it is unsafe to assert that 

 a deposit poor in arragonite shells was formed at a time when, 

 or in an area where, few such shells existed, and as it is not 

 necessary to suppose that there were no lichens, or mosses, 

 or grasses, or sedges in a given period of the past, simply 

 because its deposits contain no fossils of these groups, it 

 may be very far from certain that there were no men during 

 what is called the cavern era, even if it be a fact that no 

 human bones have ever been found in the " cave-earth." 



Whilst it is true that the bones of the human and of the 

 infra-human mammals consist of the same materials, it must 

 not be supposed that the different bones of even the same 

 animal, or different parts of one and the same bone, or cor- 

 responding bones in individuals of different species, are iden- 

 tical in composition. They differ in the proportions in which 

 the components are mixed ; or, in the language of the 

 chemist, though they agree qualitatively, they differ quanti- 

 tatively. We must refer our readers desirous of full informa- 

 tion on this point to the analyses of Von Bibra,t from which 

 we have compiled the annexed table (see next page), showing 

 the composition of the thigh bone in the human subject at 

 different ages, and in different species of the lower mammals. 



Though it may be true that in Egypt no difference is 

 observed between the condition of the mummies of men and 

 of quadrupeds, it does not appear that the fact has much, 

 if any, bearing on the question before us, for the human 

 bones we are supposed to be in quest of could not have been 

 undenconditions at all similar. They were neither placed 



* See Report of British Association, 1862; Proc. of Sections, p. 95. 

 f See Day's Simon's Chemistry, vol. ii., Pt. ii., pp. 396 et seq. 



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