118 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [JuilC 17, 



Amongst these, the following species may be especially mentioned 

 as either by their presence or quantity marking this stratum : — 



Apiocrinus rotundus, Park. Rhynchonella media, M. C. 



Serpula grandis, Goldf. Avicula costata, M. C. 



Terebratula dig'ona, M. C. Cardium gibberuluin, Phil. 



coarctata, M. C. Pecten vagans, M. C. 



cardium, M. C. Astraea, species. 



7. Forest Marble. 



Though somewhat irregular, both in structure and thickness, this is 

 tolerably persistent over a wide tract of country in South Gloucester- 

 shire and North Wilts ; in the neighbourhood of Cirencester nearly 

 all the heights are capped with this stratum ; and, as the town rests 

 in a valley of depression, to be more fully explained hereafter, it will 

 be seen that Forest Marble Clays are the water-bearing beds of the 

 town, as shown in the sections through Cirencester. 



This stratum consists of sands, clays, and cherty limestones, with 

 occasional thin bands of a tolerably fine and much ripple-marked sand- 

 stone ; the latter, however, being only what in other places are loose 

 sands, have become indurated from some local and peculiar chemical 

 cause ; a quarry on the Somerford road presents the following sec- 

 tion : — 



ft. in. ft. in. 



1. Forest Marble clay 3 9 3. Bed of fine sand, including 



2. Forest Marble sandy tile- "potlids" 8 



stones 4 



These "pot-lids" or nodules, which sometimes assume most curious 

 shapes, split up into tiles thin enough for roofing-purposes ; they 

 seldom include fossils of any kind, and indeed the siliceous beds of 

 the Forest Marble are very free from shells. 



The ripple-marked sandstones are exceedingly interesting as afford- 

 ing evidence that so many of the circumstances attendant upon a 

 modern sandy beach were in full operation when these beds were 

 being deposited ; the irregularities on the surface of the stone are the 

 result of the consolidation of the " ribbed sea-sand," and was, as now, 

 due to the tidal wave, or arranged by the action of the wind. The 

 powdering of these stones with a carbonaceous dust, and the indenta- 

 tions caused by the tracks of Crustacea, Conchifera, and Annelides, all 

 plainly indicate the past condition of the sea-shore, and enable us to 

 compare the past with the present with great exactitude. 



The most common disposition of the upper beds of the Forest 

 Marble may be gathered from the following sections, — premising, 

 however, that each section will differ in details ; what is sand at one 

 place may be sandstone at another, while shales and marls are re- 

 placed by impure limestone. 



Section, Pool Road Bridge on Great Western Line, 4 miles from 



Cirencester. 



ft. in. 



1 . Cornbrash 4 



2. Forest Marble clay 3 



