Vol. 51.] FORMATION OP OOLITE. 197 



layers were really made up of very minute tubules, which I identi- 

 fied as the little-understood and obscure organism Girvanella. 



This discovery was communicated to the Geological Magazine in a 

 paper on ' The Microscopic Structure of tbe Jurassic Pisolite.' * Since 

 then I have communicated several papers to the Geological Society 

 in which the origin of oolitic granules has been touched upon ; and 

 I have shown that not only are the pisolites formed by the growth 

 of Girvanella-tnbvles around a nucleus, but that some of the smaller, 

 or true, oolitic granules have a like origin. Further, I have myself 

 long entertained, and privately expressed the opinion that oolitic 

 granules in general are of organic origin, but up till now I have not 

 been in a position to prove this. The radiating structure and the 

 apparently endless concentric layers were features which I could not 

 explain. My failure to arrive at the truth was due to a mistake 

 which most of us are liable to fall into ; it is that, when we examine 

 a number of similar objects under a microscope, our attention is 

 naturally directed to what we consider to be the most perfect speci- 

 mens. Thus, in the case of oolitic granules, we select for examination 

 those in which concentric layers are best defined. The less inter- 

 esting granules, in which the concentric structure is only partially 

 seen or is absent, are liable to be disregarded. I shall, however, 

 show that they are really the most interesting, as affording evidence 

 which enables us to arrive at the origin of the granules. 



II. Forms of Oolitic Granules. 



In PI. VII. fig. 1 is represented what I may term the simplest 

 form of granule. The object occurs in a slide of Carboniferous 

 Limestone collected near Buxton, and was given to me by Mr. Edward 

 Wilson, F.G.S., Curator of the Bristol Museum. 



In the centre of the granule is a nucleus consisting of an aggre- 

 gation of Girvanellce, and this is enclosed by a crust. The Girvanella- 

 tubules appear in section ; that is to say, they have been sliced 

 through in the process of grinding down the slice of limestone. 

 Thus on either side of each tubule we see a thin dark line which 

 represents the calcareous wall, but the central portion is quite trans- 

 parent, owing to the fact that on the decay of the organic matter 

 the tubules have been filled in with calcite. 



Coming to the crust around the nucleus, it is seen to be made up 

 of clear and dark irregular laminae concentrically arranged around 

 the nucleus, and my object is to determine whether this crust is of 

 organic or inorganic origin. 



Some light is thrown on the matter by the object marked A 

 (PI. VII. fig. 1). There we have a long tubule-like body bounded on 

 either side by a thin dark line, and the interstitial space is trans- 

 parent. This object, then, resembles the Girvanellce in the nucleus, 

 except that it is much more extended and has a decidedly concentric 

 growth. In parts of the crust similar objects are to be seen, but in 



1 Geol. Mag. 1889, p. 196. 



