506 THE WONDERS OF GEOLOGY. Lect. V. 



the sea ; whilst the western ranges from 600 to 800 feet, and the cul- 

 minating point, Cleeve Cloud, is 1,134 feet high. 



" The branch of the Great Western Railway from Swindon to Glou- 

 cester passes through the centre of the district, and affords a key to 

 its geological structure. 



" These hills are entirely composed of two series of oolitic limestones, 

 separated by a bed of clay known to geologists as the ' Fuller's earth.' 

 The strata are inclined to the S.E., at the rate of about 1 in 130, or 

 less than half a degree ; yet this inclination is greater than that of the 

 general surface, and sufficient to carry the hill strata beneath the 

 newer formations on the S.E. ; whilst on the N.W. boundary they 

 terminate in steep escarpments, that are broken and indented by nu- 

 merous deep and picturesque valleys. 



" In order to obtain a general idea of the nature and succession of the 

 strata of the whole district, we may conveniently pursue the well- 

 known Roman road termed the ( Irmin Way,' which coming from 

 Newbury through the Wanborough downs, runs almost in a straight 

 line to Cricklade, Cirencester, Birdlip, and Gloucester, and traverses 

 in succession the whole of the oolitic strata, in the following order : — 

 "1. Portland stone. 2. Kimmeridge clay. 3. Coral rag. 4. Ox- 

 ford clay and Kelloway rock. 5. Great Oolite : subdivided 

 into Cornbrash, Forest marble, Bradford clay, Bath freestone, 

 Stonesfield slate. 6. Fuller's earth. 7. Inferior Oolite. 8. Lias. 



" 1. The Portland stone is extensively quarried near Swindon Old 

 Town. 



" 2. The Kimmeridge clay forms the valley in which the Swindon 

 station and the New Town are situated. 



" 3. The Coral rag rises up from beneath the cla} T , and constitutes 

 the hills about Stratton, Saint Margaret's, Pen Hill, and Blunsdon. 



" 4. The Oxford clay occupies the whole of Braydon Forest, and 

 the wide valley around Cricklade, but is covered in many places by 

 thick beds of oolitic gravel. 



" 5. At Driffield cross-ways the Cornbrash is seen in the small 

 quarries from which the road-stone is procured ; and in descending the 

 hill towards Cirencester we pass to the Forest marble, a thin-bedded 

 stone, well shown in the quarries at Preston, from which much of the 

 roofing-stone and planking, so extensively used in the neighbourhood, 

 is obtained. A bed of clay is usually found separating the Forest 

 marble from the Bath freestone, and is therefore the equivalent of the 

 Bradford clay. Beyond Stratton the road lies over the Bath freestone 

 (or 'Hampton stone') as far as Highgate, where several deep valleys 

 expose the Stonesfield slate, Fuller's earth, and Inferior Oolite. 



