Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/370

 " I have also collected a similar mineral substance occurring on the surface and filling the crevices of the white oolite of Lincolnshire, where this rock is covered by the soft shales, sandstones, and clays belonging to this series. It has probably originated in the same manner as scarbroite and allophane, by the decomposition of some of the mineral substances of the overlying strata."

No analysis is given of this mineral ; but, from its geological position and mode of occurrence, it resembles the earthy substance found by Dr. Berrill in the Northampton Sand, although this last, as will be seen from the following tabulated analyses, agrees much more nearly in composition with halloysite or samoite than with pholerite. Name. Locality. SiO 2. Al 2 O3. H 2 O. Analyst.

Pholerite " Samoite " Halloysite (dried at 100° C.) Earthy mineral (dried at 100°C.)

Fins. Darlaston. Anglar. Northampton. "

41.65 41.78 31.25 35.14 39.50 44.94 35.19 43.92 43.35 36.99 37.21 31.95 34.00 39.06 32.22 40.22 15.00 14.26 30.45 30.80 26.50 16.00 29.87 15.86 Guillemin. Dick. Silliman. " Berthier. " The Author. "

Fe 2 O3 4.51, MgO 0-16, Ca 0.48.

MgO 06, NaO .06, CaCO 3 .01.

MgO 1.05, NaO trace, CaCO 3 1.21.

Fe 2 O 3 .02, FeO.28, CaOl.72, MgO .03, C0 2 .39, S0 3 trace.

In the analysis of the earthy mineral from Northampton the loss due to the expulsion of carbonic acid was deducted from the loss on ignition, and the increase in weight arising from the oxidation of the ferrous oxide was added to the water fixed at 100° C.

On the supposition that the mineral consists essentially of water fixed at 100° C, silica, and alumina, it would be represented by the formula

8A1 2 O3 15Si0 2 +18H 2 0;

but if the water that is given off at 100° be considered essential to its composition, it would be expressed by

Al 2 O3 2Si0 2 +5H 2 0.

The mineral dried at 100° is exceedingly hygroscopic, speedily regaining almost the whole of the water it had lost, and that too in well-ground, tightly fitting watch-glasses.

Discussion.

Mr. David Forbes stated that he had found phosphoric acid in the first-mentioned mineral, which was perhaps the cause of its lustre. The mineral was probably not pure allophane.

Prof. Morris suggested a chemical and microscopical examination