Vol. 51.] FORMATION OP OOLITE. 201 



explain the want of continuity and the consequent vacant spaces 

 which occur. It is, however, possible that some of the structure- 

 less material present may be secreted ; but if so I regard this 

 process as due to the organic element acting in some such manner 

 as the secreting-algse. Had the tubules been enclosed by inorganic 

 substances they would be irregularly placed, but this is not the 

 case ; though there are vacant spaces, yet there is definite concentric 

 order. 



PI. VII. fig. 9. — This, too, is a type of granule not unfrequently 

 met with, and it is also interesting because of the molecular changes 

 which appear to have taken place. The nucleus has lost all trace 

 of structure, and it is surrounded by a fibrous crust showing con- 

 centric growth here and there. This crust is finally surrounded by 

 a distinct tubular form of growth, well illustrated at the point B, 

 where a large tubule terminates, and from beneath it three others 

 crop up in succession, ending in circular spots which are really 

 sliced faces of tubules. 



III. Origin op the Crusts around the Nuclei. 



Under the head of ' forms of oolitic granules,' I have here and 

 there touched upon details which might well have been discussed 

 under the present heading. This could scarcely be avoided under 

 the circumstances, but I have, as far as possible, spared repetition. 



It is, however, plain that all the granules to which I have 

 referred cannot have been formed by Girvanellce. As I have before 

 pointed out, the tubules belonging to this genus are enclosed by 

 dark walls, but in the case of granules of which PI. VII. figs. 3, 4, 

 and 6 are types, the forms of growth are ' clear ' but non-crystalline, 

 and there are no dark walls apparent. "We have, then, two distinct 

 types of granules before us. There are probably cases in which 

 the dark walls of Girvanellce have been replaced by calcite, but I 

 am disposed to regard the two types of granules under consideration 

 as originating from distinct forms of growth, and not from the 

 mineralogical changes. 



With regard to the granules represented in PI. VII. figs. 1 and 8, 

 I have already described them so much in detail that I need only 

 sum up my conclusions as to the origin of these two. They have 

 been formed by the growth of low tubulated forms of life closely 

 allied to Girvanella. 



The origin of the crusts represented in PI. VII. figs. 3, 4, and 6 is 

 more complicated. With regard to fig. 6, a portion of the crust 

 has a very tubulated appearance, but this is not continuous, a fact 

 which makes it evident that the granule is not a concretion. Had 

 this been the origin — that is to say, had the nucleus been enclosed 

 by concretionary layers of calcium carbonate — then the concentric 

 layers should be seen in all parts of the section, no matter in what 

 direction the granule had been cut through. This is not the case 

 in the granule before us. It may be argued that this structure 



