408 J. C. Mansel-Pleydell — Geology of Dorset. 



east of Sutton Pointz. The southern boundary of this zone is 

 almost a straight line from Abbotsbury to Broadwey, from whence 

 it takes a south-west direction, meeting the coast-line at Osmington 

 Mills ; at Ringstead Bay it disappears under the sea-level, its most 

 inland portion being South Hoi worth. It occupies a small area from 

 the commencement of Smallmouth Sands to the Ferry Bridge, and 

 forms the coast-line from Chesil on the west to Portland Nore and 

 Church Hope Cove on the east of the island. Its most easterly 

 extension is from Gad Cliff to St. Alban's Head. The Kimmeridge 

 Clay of the Vale of Blackmore is less bituminous than that of the 

 Kimmeridge district ; where it is upwards of 600 feet thick, and 

 strongly imjoregnated with bitumen, giving out a disagreeable smell. 

 It burns vividly with a bright and clear flame, but is unfit for ordi- 

 nary use as fuel ; the poor of the neighbourhood, however, use it as 

 a substitute for other fuel. It burns with a cheerful flame, lighting 

 up the chamber, as well as cooking the dinner and boiling the kettle. 

 It has, however, an unpleasant odour when burning, resembling- 

 petroleum, and yields a reddish ash. The volatile matter of the richer 

 beds is upwards of 73 per cent. ; the solid mineral matter being- 

 only 27. The following is the result of the analysis of J. W. Keates, 

 Esq., F.C.S., Consulting Chemist of the City of London G-asworks : 

 "Nine thousand cubic feet of gas per ton. The lighting power 

 of this gas is equal to 16 candles (sperm), consuming 120 grs. per 

 hour, being burned at the rate of 9-2 cubic feet in a fifteen-holed 

 argand burner. The lighting power of ordinary London gas is 

 12 candles similar to those mentioned above. 



Volatile matter ... 61 



Carbon or coke 39 



100 



The coke contains — Carbon ... 13-15 

 Ash ... 25-85 



39- 



The ash contains — 



Insoluble residue 29-01 Silica 2176 



Peroxide of iron 7-10 Alumina 10-60." 



The beds between Chapman's Pool and Gad Cliff are much dis- 

 located by faults which arrest the appearance of the lower beds of 

 the series, otherwise the fossiliferous stratum of Holworth would 

 have succeeded in its proper place instead of being hidden fifteen 

 miles eastward. An anticlinal axis at Kimmeridge Bay is another 

 cause of the retardation of the appearance of the beds in their proper 

 succession. The bands of cement-stone which intercalate the clays 

 and shales facilitate the recognition of the beds, and show, within 

 the space of a mile, their re-appearance no less than six times. At 

 St. Alban's Head and Gad Cliff the upper beds alone are present, 

 the intermediate space (about six miles) displaying not more than 

 one-half of the series. The lowest member of the series is at Eing- 

 stead, and, owing to the rock losing its shaly character, the fossils 

 are well preserved. The beds have a northward dip, and are, as 

 already noticed, intersected by numerous faults, causing dislocations, 

 analogous to those of the Coal-measures. The distinctive chai-acter 

 of this formation, as compared with the true Carboniferous, is, that 

 the bituminous matter is principally a result of the decomposition of 



