Yol. 51.] FOKMATIOST OP OOLITE. 203 



granule is not a concretion, that is to say, has not been formed by 

 the deposition of concentric strata of calcium carbonate around a 

 nucleus, neither can we conclude that the granule shown in fig. 5 is 

 a concretion. 



Some further light is thrown on the spots in the concentric 

 strata by PL VII. fig. 9. As I pointed out when describing 

 this granule, a large tubule terminates at B and an oval-shaped 

 section of the face is exposed. Had there been a succession of such 

 tubules cropping up one over the other, then the sliced faces of 

 these would resemble the more or less oval spots in the concentric 

 strata in fig. 5. 



To sum up my views as to the origin of the spots or openings in 

 the concentric and non-concentric portions of granules : — In the 

 concentric layers I regard them as the sliced faces of tubules 

 cropping up one over the other, and in the non-concentric as the 

 same, only seen in plan. The two positions in which they are seen 

 seem to arise from the irregular manner of growth. 



I have pointed out that there are inconsistencies between the 

 tubular growth in PI. VII. figs. 1 and 8, and figs. 3, 5, and 7. In 

 the former the tubules are enclosed by dark walls and filled in, for the 

 most part, with calcite. In the case of figs. 3, 5, and 7 the tubules 

 are clear, though non-crystalline, by reflected light, while they have 

 a somewhat milky appearance by transmitted light, and some of 

 the infilling-material is amorphous. 



In further explanation of this, I repeat that I do not say that all 

 the forms of growth which I believe have contributed to the forma- 

 tion of oolitic granules are identical with Girvanella. There are 

 probably many varieties of oolite-forming organisms. Again, 

 molecular changes do take place in fossil organic calcareous struc- 

 tures. For example, take the stromatoporoids or the sponges. 

 Prof. A. H. Nicholson observes, 1 with reference to the former, that 

 in some instances the skeleton is gradually dissolved and is replaced 

 by crystalline carbonate of lime. That a change of this nature 

 may have taken place with regard to the calcareous portion of 

 the tubulated growth in oolitic granules is not impossible, and 

 indeed seems to find confirmation in the case of PI. VII. fig. 9. In 

 this granule the larger tubule at A is traversed at right angles by 

 minute needle-like crystals of calcite, and this form of crystalliza- 

 tion probably explains the fibrous appearance in the inner crust ; 

 that is to say, the calcium carbonate composing the walls of the 

 tubules has been changed into fibrous crystals of calcite. 



I now come to the radiate structure which is so marked a feature 

 in the crust of oolitic granules. I confess that the cause of this has 

 been a difficult problem to solve, and it was long before I could 

 bring myself to accept the solution which evidence seemed to dictate. 

 I feel sure, however, that the explanation which I am about to give 

 is at any rate a partial explanation, but it is possible that crystal- 

 lization may also be a contributing cause. 



1 Palscont. Soc. vol. xxxix. (1885) ' Monogr. Brit. Stromatop.' p. 31. 



