﻿1850.] 



SORBY ON THE CALCAREOUS GRIT. 



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pied with colourless agate, and (under the former condition) do not 

 therefore exhibit their component layers. In some few rare instances 

 there is a vacant space left in the centre, owing to this last stage of 

 agatization not having occurred ; and often the whole is filled with 

 homogeneous agate. There are also cases where there is more or less of 

 granular impurity in them, either disseminated or collected together. 



But not only are these reniform bodies converted into agate, but 

 also very frequently are entirely filled with calcareous spar. The 

 difference between agate and calcareous spar is very readily discover- 

 able by their action on polarized light. Calcareous spar, owing to 

 its very intense double refraction, does not give rise to colours in 

 the same manner as quartz or agate, in which the double-refracting 

 power is so much less ; but except when we see portions of its system 

 of rings, the only action which it has on polarized light, is to depo- 

 larize it in some positions as white light, and by rotation definite 

 neutral axes are found. Thus, if the whole of the reniform body is 

 filled with calcareous spar, having the crystalline particles all ar- 

 ranged in one direction, by rotating it round the beam of polarized 

 light, with the analyser in such a position as entirely to suppress it, 

 in some positions white light will be seen to pass through the whole, 

 while in others the body will appear entirely dark. In other cases 

 the bodies are filled with calcareous spar, which has been crystallized 

 in portions that have their neutral axes inclined to one another at va- 

 rious angles, so that by rotation, one portion is dark, whilst others are 

 light ; which by farther rotation are easily shown to be unconnected 

 in their crystalline arrangement. When however they are filled with 

 agate no such effect is produced, but in every position we have the 

 appearance of a circular white disc, with black, blue and orange 

 bands radiating in an irregular manner from the centre, and changing 

 places by rotation. In some cases, one portion is converted into 

 agate, whilst another is calcareous, sometimes in the form of crystal- 

 line spar, and at others as a concretion, similar to those sometimes to 

 be seen in the substance of the shells from the grit, which are partially 

 agatized. In these, some of the original calcareous matter still re- 

 mains in the form of small concretions, sometimes disseminated in 

 the agate, but generally attached to the sides of the shell. In the 

 same slice, and in the same field of view of the microscope, we find 

 some of the reniform bodies filled with agate, and others with calca- 

 reous spar, but their relative numbers vary in different parts ; in some 

 parts nearly all being agate, and in others calcareous. 



I counted in the space of a-g^-th of a square inch, in an average 

 portion of a thin slice, no less than forty of these bodies, and there- 

 fore on a square inch there would be 11,200; and since they are 

 on an average i^-th of an inch in diameter, there would be at this 

 rate about two and a quarter millions in a cubic inch. Moreover, 

 taking into consideration those filled with calcareous spar as well as 

 those that are agatized, I shall not be overrating them if I suppose 

 them to constitute 20 per cent, of the whole rock. Calculating on 

 this supposition, and on their being T ^th of an inch in diameter, 

 and therefore about 15 ^ 0OQ th of a cubic inch in content, there would 



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