I48 SELENITE. 



charged with sulphate of lime far more heavily instead 

 of much less so, than the London-Clay waters, seeing the 

 ample natural supply of the calcareous element. The boulder- 

 clay land is improved by lime-dressing, where the surface-soil has 

 been impoverished by the removal ^of its calcareous matter, but if 

 any sulphurous rainwater can percolate into the mass of the 

 boulder-clay (which Mr. Dymond holds to be permeable^), there 

 should be far more abundant crystals of selenite formed there 

 than in the almost limeless London Clay; but none such are 

 to be found. 



That the London Clay requires lime-dressing is prima facie 

 evidence of the normal acidity of the clay when the finely- 

 divided pyrites (to which the blue colour is due) has been 

 oxidised by atmospheric action into iron sulphate and free 

 sulphuric acid, which in the absence or deficiency of lime or 

 alkali has to remain unneutralised. 



I would therefore answer Mr. Dymond's closing query in the 

 full negative ; it is not conceivable that the selenite in the 

 London or any older clay derives either its lime or its sulphur 

 from external sources ; it owes a part only of its oxygen to the 

 air, and its water of crystallisation to the rain ; the important 

 constituents are native to the deposit. If I mistake not, selenite 

 nowhere occurs in superficial deposits, nor in unweathered and 

 absolutely impervious clays. The action of water charged with 

 oxygen is essential to the disruption of the pyrites, but it seems 

 necessary for the production of crystals that the supply of the 

 ingredients be slow, and under conditions which as yet we have 

 not been able to determine. 



A totally different problem is presented by the sulphate of 

 baryta, constituting the crystalline lining of some septaria. In 

 what condition was this almost insoluble mineral or its con- 

 stituents, at the time of the contraction of the nodule, and 

 the infilling of the radiating and concentric fissures resulting ? 

 The recently-detected occurrence, in a Lias septarium, of zinc- 

 blende is a kindred puzzle. 



NOTE BY MR. DYMOND. 



While I must defer to Mr. Dalton's opinion that selenite 



nowhere occurs in superficial deposits of the London Clay, I do 



not think the arguments he adduces against my suggestion 



" that a part of the sulphate of lime that hardens the water 



