﻿1850.] 



SORBY ON THE CALCAREOUS GRIT. 



5 



transparent ; but this transparent part was not so thick as the shell 

 originally was, and in some cases did not occur at all. The septa, 

 however, being much thinner than the exterior shell, could not be 

 detected in the agate. These facts therefore show, that though we 

 cannot detect any trace of shell in the reniform bodies which are 

 filled with agate or calcareous spar, we are not warranted to conclude 

 that it did not at one time exist. On this account I very carefully 

 examined a thin section of considerable size, in order to ascertain 

 whether there existed indications of any of these bodies having been 

 burst in, and whether the arrangement of the impurities introduced 

 by that means afforded evidence of the former existence of a shell. 



Although such cases are rare, yet I have found some which unques- 

 tionably proved that they had been broken, and from the manner in 

 which the impurities occur, I think that the existence of a shell is 

 rendered very probable. In one instance the section of an agatized 

 reniform body showed that it had been ruptured at one end, and 

 the impurities introduced by that means into the interior are col- 

 lected in such a manner as clearly to indicate the existence of a shell; 

 a distinct uniform line or space of clear agate occurring between the 

 outer boundary of the reniform body and the extraneous granular 

 matter that partially occupies its interior. In another instance, a 

 section of a calcareous body exhibits a thin layer of fine granular 

 matter following the curvature of the inner surface of the reniform 

 body, and lying within, and at a slight uniform distance from its 

 outer boundary ; whilst a straight elongated fragment lies within, 

 and rests its ends against the line of granular matter. Hence it 

 appears that both the finer matter and the larger extraneous frag- 

 ment rested originally on the inner surface of a shell that has since 

 been removed. The thickness of the shell shown by both these spe- 

 cimens was about y^^th of an inch, which is so thin, that we could 

 not expect to find any trace of it in the agate, when the septa of the 

 ammonites are obliterated. These facts, I think, indicate that these 

 bodies were small shells, whose interiors have been filled with calca- 

 reous or siliceous infiltrations, in the same manner as the chambers 

 of the ammonites which are found in the rock. Nevertheless, I will 

 not insist on this view, for I have found cases where the impurities 

 were not arranged as though there had been a shell ; but on the 

 whole, I should say, that there is better reason for thinking them 

 to have been shells than any other bodies with which I am ac- 

 quainted. If really shells, they may perhaps have been Foramini- 

 fera, although I have not been able to detect any internal divisions 

 into chambers, nor anything to indicate that they are detached fora- 

 miniferous cells, such as are sometimes met with. 



Besides the reniform bodies, we find in the matter left undissolved 

 by acid several other minute agatized organic bodies, which it may, 

 perhaps, be as well to describe. There occur minute cellular bodies 

 (zoophytes ?), formed of more or less oval, open cells, concave on one 

 side and convex on the other, placed end to end, and communicating 

 at the extremities, in a similar manner to what is seen in the section 

 of a Nodosaria. The cells are frequently irregular and vary much 



