196 MR. E. B. WETHERED ON THE [May 1895, 



17. The Formation of Oolite. By E. B. Wethered, Esq., F.G.S., 

 F.C.S. (Read January 9th, 1895.) 



[Plate VII.] 



Contents. Page 



I. Introduction 196 



II. Forms of Oolitic Granules 197 



III. Origin of the Crusts around the Nuclei 201 



IV. Process of Formation of Oolitic Granules 204 



I. Introduction. 



If a thin section of oolite be mounted in Canada balsam and examined 

 under a microscope, it is seen to be made up of a number of granules 

 associated with fragmental remains of calcareous organisms, and the 

 interstitial spaces are filled in with calcite. 



Looked at with a 2-inch object-glass these oolitic granules show, 

 for the most part, a series of concentric strata or laminae of calcium 

 carbonate around a nucleus. There are also other secondary struc- 

 tures, the principal of which are dark stria? and patches, the former 

 placed more or less at right angles to the nucleus. 



At the Bath meeting of the British Association in 1888 Prof. H. 

 G. Seeley read a paper ' On the Origin of Oolitic Texture in Lime- 

 stone Bocks.' In this communication the author stated that he 



' believed that oolitic texture might originate in many ways ' 



He also drew attention to the ' close resemblance of the internodal 

 grains of the nullipores to grains of oolite as furnishing a further 

 explanation of oolitic texture. These grains show a con centric 

 structure as well as a radiated tubular structure, which would favour 

 the recrystallization such as commonly occurs.' 1 



Generally speaking, up to the year 1889 the concentric layers 

 were supposed to be chemically deposited calcium carbonate. Thus 

 Dr. Sorby, F.R.S., in his most able and memorable address to the 

 Geological Society in 1879, describes the formation of normal Jurassic 

 oolitic grains in the following words : They are ' composed of more 

 or less well-defined layers of small crystals of calcite ...... These 



crystals are of such an uniform size, both in the same and in different 

 grains, and so definitely oriented, that I can scarcely attribute them 

 to the recrystallization of aragonite, and must conclude that oolitic 

 grains of this type indicate the original deposition of calcite round 

 nuclei gently drifted along by currents of the ordinary temperature, 

 which caught up more or less of the surrounding mechanical im- 

 purities.' 2 



In the year 1889 I happened to have under examination some 

 pisolite from the neighbourhood of Weymouth, kindly sent me by 

 Mr. H. B. Woodward, E.G.S. I then discovered that the concentric 



1 Eep. Brit. Assoc. 1888 (Bath meeting), pp. 674 & 675. 



2 Quart. Journ. Geol. fcioc. vol. xxxv. (1879) Proc. p. 75. 



