236 



PROCEEDINGS OF THE GEOLOGICAL SOCIETT. [Feb. 22, 



" I have also collected a similar mineral substance occurring on tbe 

 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 posi- 

 tion 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 haUoysite or samoite than with pholerite. 



Name. 



Locality. 



SiO, 



AloO, 



H,0. 



Analyst. 



Pholerite 



>) 



Samoite 



j» 



HaUoysite 



(dried at 100° C.) 

 Earthy mineral 



(dried at 100° C.) 



Fins. 

 Darlaston. 



Anglar. 

 Northampton. 



41-65 

 41-78 



31-26 



35-14 



39-50 

 44-94 

 35-19 



43-92 



43-35 

 36-99 



37-21 



31-95 



3400 

 39-06 

 32-22 



40-22 



15-00 

 14-26 



30-45 



30-80 



26-50 

 16-00 

 29-87 



15-86 



Fe^ O3 4-51, MgO 

 0-16, Ca 0-48. 



MgO -06, NaO -06, 

 CaCOg -01. 



MgO 1-05, NaO 



trace, CaCO, 1-21. 



Fe„O3-02, FeO-28, 

 CaO 1-72, MgO 

 -03, CO2 -39, 

 SO, trace. 



GuiUemin. 

 Dick. 



Silliman. 



Berthier. 

 The Author. 



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 



8Al,03l5SiO,+18H,0; 



but if the water that is given off at 100° be considered essential to 

 its composition, it would be expressed by 



Al3 03 2SiO, +5H,0. 



The mineral dried at 100° is exceedingly hygroscopic, speedily re- 

 gaining almost the whole of the water it had lost, and that too in 

 weU-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 



