330 Geology of WilMire. 



Forest Marble. 



All the strata between the white lime-stone and Cornbrash have 

 been mapped as Forest marble, marked y^ ; the white limestone 

 forming a constant and easily recognized base upon which the 

 variable strata of the Forest marble have been deposited. This 

 formation includes shelly fissile oolite, in which false bedding is 

 exceedingly prevalent, together with flagstones, sandy slates, clays, 

 and siliceous sands. 



On comparing the eastern portion of the district occupied by 

 Forest marble with the western, it would appear that the fissile 

 oolite of the one occupy the position of the clays and flags of the 

 other. Thus, along the Great Western Railwa}', the fissile oolite 

 rests on the white limestone, and is succeeded by bluish clays and 

 limestones, shown in the cutting south of Kemble, while on the 

 other hand, east of Cirencester, we find clays and flags resting on 

 the white limestone, and succeeded by fissile oolite, upon which 

 rests the Cornbrash. This will be apparent on comparing the 

 position of the beds in the quarries at Amney with those shown 

 in the Roman road three miles east of Cirencester. Thus we 

 have the clays of one district occupying the position of oolite in 

 another, and vice versa. 



Some of the bluish flagstones of the Forest marble present a 

 curious collection of fragmentarj' fossils, in which we find shells 



de-viation from a straight line, was only one inch and a quarter ! This in a 

 cutting of 1520 feet in length, begun at opposite ends, and worked towards a 

 common centre, is perhaps unexampled in the annals of tunneling. Notwith- 

 standing the unfortunate accidents with which Messrs. Brewer & Lewis had 

 to contend, the obstacles which crossed their path only incited them to greater 

 efforts to comj)lete their undertaking, and in order to make up for the delay 

 occasioned by the irruption of water they employed upwards of 300 workmen. 

 The cutting, on the Corsham side, extends through one solid bed of freestone 

 or superior Oolite, in many places 130 feet thick, and lying upon a bed of 

 fuller's earth, or clay, 120 feet in thickness; under which blue marl, resting 

 upon lias clay is found. So uninterrupted and compact is the rock through 

 which this end of the tunnel passes, that no masonry was required in any part 

 of it, the stone itself forming sides and roof, and nothing being required at the 

 bottom but the rails on which the carriages run. One ton of gunpowder was 

 used every week in blasting this stone, and thirty-live tons of coal were also 

 used in the same time. 



