222 R. R. Chandler — A Bej^osit of AUojjhanc. 



Fragments of Pembrokesiiire rocks, glacially transported, have been 

 traced as far east as Glamorganshire ^ ; moreover, the shore drift 

 contains in places abundant flint for which Ireland itself is not an 

 impossible place of origin. 



I am much indebted to Mr. J. V. Elsden and Mr. H. H. Thomas for 

 kindly identifying the rock-sections of the boulders with their parent 

 masses, and to Mr. A. L. Leach for chips of the Lydstep Boulders. 



YIII. — Note ok a Deposit oe Allophane. 

 By R. H. Chandler. 



IN a denehole at Abbey Wood, near Plumstead, there is shown 

 in the Chalk a fault of unknown extent (but probably having 

 a throw of only a few inches), the plane of the fault roughly 

 coinciding with one of the chamber walls. This wall has a strongly 

 slickensided surface for some superficial yards, and extends to the 

 roof of the chamber where the deposit of allophane is contained 

 between the two fault faces. In some instances this mineral may 

 be seen adhering to the fault face, and the roof of the denehole is 

 composed of it for a length of about 7 feet by 2 feet wide ; other 

 patches are to be observed where the roof has been broken away, so 

 that it seems evident the allophane extends horizontally some distance 

 between the Chalk and the Thanet Sand ; this latter is about 32 feet 

 thick here. The deposit appears to be a roughly wedge-shaped 

 mass, the greatest width being at the top of the Chalk, and from 

 there diminishing downwards into tongues of the mineral extending 

 between the two boundaries of the fault. Mr. J. L. Eoucar, B.Sc, 

 F.C.S., who kindly submitted a specimen to analysis for me, says: 

 * ' I find the following to represent its composition : — 



Moisture (loss at 100° C.) 23-19 per cent. 



Loss on ignition, including some C03 not separately 



determined . . . . . . . 20'77 



Si02 . . . 28-80 



AI2 O3 23-52 



Ca 2-45 



Loss and undetermined . . . . . 1-27 



Fe . . . traces 



100-00 ,, 

 He adds: "The mineral is undoubtedly a variety of allophane, 

 although its composition does not correspond with the formula 

 AlgSiOgSHgO. It has recently been shown " that the minerals of 

 the allophane group are probably merely amorphous mixtures of 

 hydrated oxides of silicon and aluminium, plus certain impurities. 

 The chief impurity in this present case is calcium carbonate, and 

 perhaps calcium silicate, the source of whose origin is obvious." 



In colour the mineral is cream to buff, veined with a darker shade 

 of brown having a waxy lustre, is rather friable, occasionally 



1 F. T. Howard & E. W. Small, Trans. Cardiff Nat. Soc, 1899, vol. xxxii ; 

 also Geology of the South Wales Coalfield, pt. viii, Mem. Geol. Surv., 1907. 



2 H. Stremme, Ceutr. Miu., 1908, pp. 622-32, 661-9. 



