926 W. H. HUDLESTON ON THE CHEMICAL COMPOSITION OF 



calculated as an oxide. Sterry Hunt had previously detected it in 

 the serpentines of Cornwall, Banffshire, and the Vosges, but states 

 that none exists in the ophiolites of the Laurentian rocks of 

 Canada*. 



Red Serpentine from the Balk quarry. — Dull purple, ground 

 streaked with green, and containing crystals somewhat similar to 

 the last, but of a green colour and more blended with the general 

 mass. Sp. gr. 2-561. 



Though so different in appearance, this rock is in composition 

 singularly like the preceding one. 



The undecomposed silicate is less in amount, and the water is 

 greater (13*28) ; the relative proportion of silica and magnesia about 

 the same; but there is only 1-37 per cent, of ferrous oxide, the 

 total iron remaining nearly the same. The bulk of the ferric oxide 

 occurs probably as haematite, dispersed in fine powder through the 

 mass, just like the magnetite in the last example f ; in this case the 

 powder does not adhere to the magnet. There is about the same 

 quantity of nickel, rather more alumina; but, on the whole, we have 

 every reason to suppose that we see here a rock whose original 

 composition was very close to that of the black rock. The gradual 

 oxidation of the magnetite into haematite, and a slightly more com- 

 plete hydration of the original basic minerals, constitute the real 

 difference between them. 



The Brass-yellow Crystals. — These lose 9 per cent, in ignition 

 (dried at 100° C), and, although composed principally of silica and 

 magnesia, contain more alumina and lime than the matrix, and 

 much less iron. Their chemical composition is similar to that of 

 bastite ; and they are probably the result of the hydration of a variety 

 of enstatite $. 



Gkoup B. 



The following are very rough analyses of some of the rocks asso- 

 ciated with the serpentines, and which, as before stated, present 

 such a remarkable contrast to them in the amount of lime and 

 alumina which they contain. This is due to the presence, in all or 

 nearly all of them, of a kind of alumina-lime felspar (?), whose 

 varieties may be grouped under the general term " Saussurite." The 

 amount of alkali in those specimens examined appears to be so small 

 that it has been queried in some cases when making up the total 

 constituents. 



One of the most interesting rocks is a peculiar gabbro occurring 

 near Caerleon Cave§. This gabbro is remarkable for crystals of dial- 

 la go of great size, associated with an opaque white mineral mass, 



* Dana's ' Mineralogy,' p. 468. 



t This is confirmed by microscopic examination (see p. 921). — T. Gr. B. 



\ See the description of the microscopic appearance, p. 921. 



§ Mr. Hudleston's specimen is very similar to some of the varieties found at 

 the Balk, so this may be taken as a type of the " Saussurite '" in this series of 

 gabbros.— T. G. B. 



