BRITISH ASSOCIATIOX MEETIXG. 



803 



tlie opiuious of Pittou, Conybeare, aud more recently of Professor Pliillips, 

 wlio on a previous occasion liad stated reasons strongly in favour of the 

 TTealden age of these beds, the course adopted by the geological surveyors 

 could only be considered provisional. 



OX THE BLEXHEBI IROX-OEE AXD THE THICKNESS OF THE 

 FOmiATIOXS BELOW THE GEEAT OOLITE AT STOXESFIELD, 

 OXFOEDSHIEE. 



By Edward Hull, B.A., F.G.S. 



The economic importance of the Liassic and Oolitic iron-ores is yearly on 

 the uicrease, OTring to three causes — ^the expansion of the British iron-trade ; 

 the local curtailment in the supply of the clay ironstone of the coal-measures ; and 

 the extension of the railway system, which has rendered available iron-ores far 

 removed from the boundaries of the coal-fields, and which were almost unknown 

 till within the last few years. Prom the '''Mineral Statistics of Great Britain," 

 collected by Mr. Hunt, it appears that in 1S57 the quantity of ore raised from 

 the Cleveland, TThitby, and Xorthamptonshire districts reached the amount of 

 rearly one and a-lialf million of tons, or nearly one-tenth of the total quantity 

 raised in Great Britain. It may safely be predicted that ere long Oxfordshii-e 

 win. also rank as an iron producing comity. 



Blzxhei:*! Irox-Ore. — The existence of highly ferruginous beds in the 

 dii'ection of Banbury and DedcUngton has been kno-wn for some years back, 

 and they have to a small extent been quarried for smelting. There are two 

 varieties, a siliceous ore, occurring at the top of the sands, which form the lower 

 zone of the Great Oolite, and a calcareous ore, forming the upper rock-bed of 

 the marlstone, or Middle Lias. During the progress of the Geological Survey 

 in the neighbourhood of Woodstock, the existence of this latter ore was ascer- 

 tained in several places, but in particular along the valley of the Cherwell, 

 west of Charlbuiy.* 



Geological position. — The Blenhenn ore is identical in geological position and 

 almost in its nature with the Cleveland ore of Yorkshire. It forms the rock- 

 bed at the top of the ]\Iarlstone, which in Gloucestershii'e and elsewhere pro- 

 duces the tabulated promontories which jut out from the flank of the oolitic 

 escarpment. At Pawler it rests upon soft sands, comprising the lower division 

 of the Marlstone, and is surmounted by the clay of the Upper Lias. It varies 

 in thickness from ten to fifteen feet, and is of nearly uniform composition through- 

 out, except where there occur bands of fossils, with an excess of carbonate of lime. 

 The shells are Marlstone species, as RJii/nchoiiella tetrahedra, Terehratiila punc- 

 tata, &c. 



Jlineral character. — At the outcrop the rock presents a rich ferruginous 

 aspect, but when reached at positions where it has been protected from 

 atmospheric influences, its colour is deep olive green ; and the gradual change 

 may be observed in blocks newly spHt. In its latter state it appears to be 

 oolitic under the lens. 



The character of the ore, before oxydization is probably that of carbonate 

 and silicate of iron, the latter imparting the green tinge : when exposed, it 

 parses into a liydi-ated peroxide of iron. The quantity of silica is about 12 per 

 cent., and of bme 10 per cent. Phosphoric acid is only present in minute 



* As this ore extends under the property of the Duke of Marlborough, I have called it 

 "Blenheim ore ;" and for fiu'ther details refer to the "Geologj- of the country roimd Wood- 

 stock." Mem. Geol. Siu'vey: 1S57, 



