Dr. FiTTON on the Strata below the Chalk. 



263 



The Upper green-sand, from Wroughton to Warborough, includes dark-coloured siliciferous 

 stone, containing mica and green particles. The formation occupies a slightly prominent step 

 beneath the sloping escarpment of the lowest chalk. Its thickness is probably between 30 and 50 

 feet (see the Section, PI. X. a., No. 17.) ; and it may be traced throughout a space of very irre- 

 gular outline from the road at Burdrop Wood to Liddington. 



Gault. — Bluish clay, of a smooth plastic texture, with numerous specks of mica, succeeds the 

 last formation, and occurs at considerable heights in the sloping face of the escarpments. It can 

 be recognised in the lower ground, on the roadsides, from Swindon to Burdrop Park, and to 

 Liddington ; but does not here form so distinct a valley or depression, as in Kent and Sussex. 



Lower green-sand. — This, though perfectly distinct, can scarcely be said to form a ridge, as in 

 the south-eastern counties, after rising from beneath the gault. It does, however, occupy a 

 slight wave or inequality of the surface, about midway between Liddington and Coate (see the 

 Section, PI. X. a. No. 17.); and then suddenly thinning off, caps the heights at the latter place 

 and at Swindon. The sand at the upper part is very tough, either from the presence of clay, 

 or a great abundance of oxide of iron. In some other places it is loose and white ; and among 

 the substances found within its range is Fuller's-earth. 



This ferruginous sand, as it approaches Swindon, rests, in several places immediately upon 

 another sand of a very different character, at the top of the Portland series, and the junction is 

 deserving of notice, the surface of the latter sand being very irregular, as if from erosion by 

 water, and the inequalities or prominences often of such tenuity, as to indicate a very tranquil 

 condition of the fluid beneath which they were formed, as well as of that which deposited t!ie 

 ferruginous mass by which they are at present enveloped : — which is the more remarkable, when it 

 is considered that the interval between the deposition of these two sands must have been, in point 

 of time, sufficient for the accumulation of the whole of the Weaklen group. 



In one of the quarries, near the top of Swindon Hill, the Section was thus. The boundary 



Ferruginous Sand. 

 (Lower Green-sand.) 



Whitish calciferous 



....■■''• Sand ; with concre- 



;. ; -y.j -./ ; ./■.■.■. • ■/■:■''■• '■,■':'■ ■ "ons in which arePort- 

 ''•",'c:^^p'.'-*'9^3^a'%(^^l';i land fossils. 







between the two kinds of sand being perfectly distinct and sharp ; and the contrast was the more 

 striking, from the lower being composed, in a great measure, of minute fragments of shells, which 

 are wholly wanting in the incumbent matter*. 



* In Mr. William Smith's Geological County Maps of Kent, Sussex, Wilts, Oxfordshire, 

 Bucks, Berkshire, and Bedfordshire, the stratum now known as the Lower green-sand is de- 

 scribed as " Sand, in the lower part of which the Portland stone is found." To understand the 

 source of this error, it must be recollected that at the period of those publications, the existence 



2m 2 



