Geological Society. 221 



Esq., employed on the Ordnance Trigonometrical Survey, and com- 

 municated by Col. Colby, F.G.S , F.R.S., &c., was first read. 



The district described in this memoir occupies a surface of about 

 1 67 square miles around Ludlow, and consists of clay-slate, transition 

 limestone, with accompanying beds of shale, old red sandstone, car- 

 boniferous limestone, the coul measures, and basalt. 



A letter from Sir John Herschel, K.C.H. to Roderick Impey Mur- 

 chison, Esq., P.G.S., " On the Cause of the Subterranean Sounds 

 heard at Nakoos, near Tor in Arabia," was then read. 



The remarks of the author relate to a communication by Mr. Greg, 

 which was read before the Society on the 27th of April 1831. He 

 suggests, as the only probable explanation which occurs to him, that 

 the pheenomena may be owing to a subterraneous production of 

 steam, by the generation and condensation of which, under certain 

 circumstances, sounds are well known to be produced. They belong 

 to the same class of phsenomena as the combustion of a jet of hy- 

 drogen gas in glass tubes. 



The author makes the general remark, that wherever extensive sub- 

 terraneous caverns exist, communicating with each other or with the 

 atmosphere by means of small orifices, considerable difference of tem- 

 perature may occasion currents of air to pass through those apertures 

 with sufficient velocity for producing sonorous vibrations. The sounds 

 described by Humboldt, as heard at sunrise by those who sleep on 

 certain granitic rocks on the banks of the Orinoco, may be explained 

 on this principle. 



The sounds produced at sunrise by the statue of Memnon, and the 

 twang, like the breaking of a string, heard by the French naturalists 

 to proceed from a granite mountain at Carnac, are viewed by the 

 author as referrible to a different cause, viz. : to pyrometric expan- 

 sions and contractions of the heterogeneous material of which the 

 statue and mountain consist. Similar sounds, and from the same 

 cause, are emitted, when heat is applied to any connected mass of 

 machinery ; and the snapping often heard in the bars of a grate af- 

 fords a familiar example of this phaenomenon. 



March 14. — A paper was read, which described, 



1st, The structure of the Cotteswold Hills and country around 

 Cheltenliam : 



2n{l, The occurrence of stems of fossil plants in vertical positions 

 in the sandstone of the inferior oolite of the Clevelartd Hillsj By Ro- 

 derick Impey Murchison, Esq., P.G.S, F.R.S., &c. 



I. Slruclurc ()f the Cottcsivold Hills and district around C/ieltenham. 



The formations constituting the Cotteswold Hills and Vale of Glou- 

 cester, in the neighbourhood of Cheltenham, are described in the 

 following descending order. 



( I .) Forest Marble, the upper members of which consist of clays, 

 containing slaty beds, the equivalents of the Stonesfield slate (Seven- 

 hampton ("ommon, &c. &c.). The lowest member of this group is a 

 liard calcareous grit, wliiili caps the hills of Lineover and Leckhamp- 

 ton, and is peculiarly distinguished by the abundance of a Gryphaa, 



