204 M.R. E. B. WETHERED ON THE [May 1895, 



In PL VII. fig. 4 we see the radiating structure in the thin crust 

 around the nucleus, together with concentric growth at both ends 

 of the granule. There is also a number of infilling or secreted 

 calcium carbonate-patches around the nucleus. Now it is certain 

 that this calcium carbonate extends, as it were, into the actual 

 crust and gives rise to the dark striae which occur. 



In PI. VII. fig. 3 we get a remarkable combination of the radiate 

 and concentric structure. 



The radiate structure has the appearance of light and dark striae 

 when seen by transmitted light. The light striae are calcitic, and 

 are really tubules which have grown at right angles to the nucleus. 



The dark striae have a different origin, and are secondary forma- 

 tions. To explain this I must again call attention to the patches 

 of amorphous calcium carbonate in touch with the nucleus. Now, 

 there can be no doubt as to the origin of this form of calcium 

 carbonate in the granule. It is either infilling or secreted. 

 Further, this same calcium carbonate extends into the crust ; some 

 of the dark striae can be traced to it, and those cannot which are 

 found to be composed of the same substance. We are therefore 

 justified in drawing the inference that the dark striae are infillings 

 or secretions. 



Such is the conclusion at which I have arrived, and the explana- 

 tion is the following. I have stated, and must repeat, that the 

 true radiate structure is due to tubules growing more or less at 

 right angles to the nucleus. Some of these have been filled in with 

 amorphous calcium carbonate and appear dark. In other cases the 

 surrounding walls of such tubules have been sliced during the 

 preparation of the slide, thus exposing the infilling material; in 

 some instances this, too, has been partially rubbed away and is 

 represented by a mere film. But the chief cause of the dark striae, 

 which are sometimes very irregular, is the infilling of interstices 

 between the true striae. 



The appearances justifying the explanation which I have given 

 are not plainly seen in all granules ; it is only in a few that the 

 real structure can be determined, Nor is this to be wondered at, 

 when we consider how frail and small are the tubular structures 

 which appear to make up the crust in oolitic granules. It is only 

 when the tubules have been filled in that the outlines are preserved. 

 When infilling has not taken place, the tubules are so pressed 

 together that it is exceedingly difficult, and sometimes impossible, to 

 trace them. 



IV. Process oe Formation oe Oolitic Granules. 



Dr. Eothpletz attributes the origin of oolitic gra.nules of the 

 Great Salt Lake to lime-secreting fission-algae. 1 In the preceding 

 pages I have frequently suggested that the amorphous calcium 

 carbonate which is present in the granules is distinct from the 



1 ' American Geologist,' vol. x. (1892) p. 280 ; transl. F. W. Cragin from 

 ' Botanisches Centralblatt,' no. 35, 1892. 



