270 MK. E. AVETHEEED ON THE OCCUEEENCE OF 



15. On the OccuEEENC]] of the Genus Gievanella in Oolitic Eocks, 

 and Eemaeks on Oolitic Steuctuee. B}^ Edwaeb Wetheeed, 

 Esq., F.G.S., F.C.S., P.R.M.S. (Read December 18, 1889.) 



[Plate XI.] 



I, Inteobfction. 



In January 1888 I commenced a systematic examination of the 

 oolitic rocks in the Carboniferous and Jurassic series. The subject 

 was, of course, by no means new, but I was under the impression that 

 there had been no systematic investigation of the structure of those 

 rocks, though many authors had referred to the subject. My 

 attention was subsequently called to Dr. Sorby's illustrated appendix 

 to his Presidential Address to this Society, 1879, and I found that 

 mAich of the work I contemplated was already done. I had, however, 

 at an early stage of the work, discovered that the little-known genus 

 Girvanella was of frequent occurrence in oolitic rocks, which appears 

 to have escaped the notice of Dr. Sorby. My work then resolved 

 itself into a search for that organism ; but, as I had to make over 230 

 thin slides for microscopic examination, I have been able to work 

 out the structure of the rocks in greater detail than Dr. Sorby has 

 done, and I trust that an account of the observations deduced from 

 those slides may prove a worthj^ supplement to his work. 



The Genus Gievanella, Nich. ^' Etheridge, jun. 



This apparently insignificant organism was first noticed by Prof. 

 H. AUeyne Nicholson and Mr. Robert Etheridge, jun., in their 

 Olonograph of the Silurian Fossils of the Girvan District in Ayrshire' 

 (Part I., 1878), and was described as consisting of " Microscopic 

 tubuli with arenaceous or calcareous (?) walls, flexuons or contorted, 

 circular in section, forming loosely compacted masses. The tubes 

 apparently simple cylinders, without perforation in their sides, and 

 destitute of internal partitions or other structures of similar kind " 

 (p. 23). The one species established was G. problematica, which 

 name was given for convenience" sake *. 



In the Geol. Magazine for 1889 f, I have shown that Girvanella 

 is not confined to the Silurian rocks, and that as a rock-forming 

 organism it is of more importance than was supposed. In that 

 communication I established a new species, G. jjisolitica, which 

 forms the Pea-Grit spherules near the base of the Inferior Oolite in 

 the northern Cottcswold area of Gloucestershire. I also showed 

 that a much smaller form built up the pisolite spherules which occur 

 in the Coralline Oolite near Weymouth ; but I did not see my 

 way to establish a third species. Since then I have obtained in- 



* See also Geol. Mag. 1888, pp. 22-24. 

 t Dec. iii. vol. vi. p. 196. 



