Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/369

 Northampton.

Charlton.

Water expelled at 100 C. " fixed at 100 C. SiO2 Al2O3 Fe203 FeO CaO MgO (Normal) CO2 (As bicarbonate) CO2

In these analyses the amount of carbonic acid was deducted from the loss on ignition ; and in Mr. Northcote's case, in addition, the carbonic acid existing as bicarbonate was deducted from the loss at 100° C.

The excess of silica in the Northampton specimen is probably due to an almost unavoidable admixture of sandstone, upon which the allophane formed an incrustation.

Mr. Sharp, in a paper lately read before the Geological Society of London, and published in the Quarterly Journal of the Society, vol. xxvi. p. 367, speaking of a section of the Northampton Sand in the grounds of the Northampton Lunatic Asylum, remarks : —

" One set of joints in this section have a direction N.W. and S.E. ; and the crevices are frequently filled with a soft white material, which, upon analysis by the recently deceased Dr. Berrill (formerly a student at the Royal School of Mines), was shown to be allied to allophane."

The substance here alluded to, a dull, whitish, pulverulent, earthy mineral, that may be polished by the nail, adhering strongly to the tongue, and exhibiting no trace of crystalline structure under the microscope, is by no means rare in many localities both in the ironstones and limestones of the Inferior Oolite of the Midland Counties.

Professor Morris, in his paper on the allophane of Charlton*, apparently alludes to a similar mineral. He says : — During the important investigations (under the direction of Dr. Percy, at the School of Mines) into the chemical composition of the different iron-ores of Britain, a white powder (probably pholerite) was observed associated with some of the clay-ironstone ; the following is its analysis by Mr. Dick:—

SiO2 41.78 ALO3 36.99 HO 14.26 Fe 2O3 4.51 CaO .48 MgO .16 Alkalies undetermined. 98.18


 * Quart. Journ. Geol. Soc. vol. xiii. (1857) p. 15.