17 



A few general Remarks on the Fossil Conchology of the Great Oolite of 

 Minchinhampton in comparison with that of the same Formation in other 

 localities. By John Ltcett, Esq. 



The following observations have been suggested to me by a remark of Dr. 

 Buckland in his " Bridgwater Treaties," and which has since been 

 occasionally quoted and repeated by others ; in effect, that during the vast 

 period when the secondary formations were in process of deposition, a 

 molluscous class (the carnivorous Trachelipods), which in our present seas 

 perform the office of keeping down within due limits the other molluscous 

 races, did not then exist, or that they were extremely few, and that it was 

 only on the exlirpation of those extensive genera of Cephalopoda, the 

 Ammonites and Belemnites, at the commencement of the tertiary epoch, 

 that the carnivorous or Trachelipods made their appearance. Living in a 

 district distinguished by a great profusion of molluscous remains, a large 

 proportion of which are absolutely unknown to science, a favourable 

 opportunity for testing the correctness of the foregoing theory was presented 

 to me, more especially as these remains occur in an unusually good state 

 of preservation, extending in some instances even to the original colours 

 of univalves, the hinges of the bivalves, and the external ligament of the 

 hinge in the latter shells. Before however stating the results of this inquiry, 

 a very brief sketch of the physical and geological characters of the district 

 may not be unacceptable to the members. 



A circle having a radius of only four miles, with the town of Minchin- 

 hampton in the centre, will comprise the whole district to which these fos- 

 sils refer. The Bath Oolite, or Compound Great Oolite as it is now termed 

 by geologists, is the uppermost formation ; its continuity is however broken 

 by two great valleys of denudation, the vales of Brimscomb and Wood- 

 chester, which, with their numerous lateral ramifications, have cut through 

 the whole series of rocks from the upper part of the Great Oolite to the 

 middle of the lias inclusive, having a mean depth of 500 feet, thereby pro- 

 ducing a combination of circumstances eminently favourable for exposing 

 the useful beds of stone and conveying it by water-carriage. 



The divisions of the Compound Great Oolite are, Great Oolite and 

 Fuller's Earth, the former having a thickness of 130 and the latter of 70 

 feet. At some few localities the base of the Great Oolite has one or two 

 beds of true Stonesfield slate associated with brown marls. In this respect 

 however, as in the mineral character of the formation generally, the greatest 

 variety and uncertainty exist; opposite sides of the same quarry will often 

 exhibit such a change ; thus an Oolitic and shelly limestone will pass into a 

 barren sandstone. Keeping this fact in view, a considerable latitude most 

 be allowed in the following arrangement, which is given only as a general and 

 approximate view of the whole series of beds. The Great Oolite proper 

 may be conveniently subdivided into three series of beds, an upper and 

 lower fossiliferous, often serviceable for building purposes, and a middle, 

 more barren and unserviceable. Beginning with the uppermost, or those 



