Cote—The Rhyolites of the County of Antrim ; with a Note on Bauxite. 107 
Styria is quoted as 2:55; and that of a sample (lithomarge?) from the Bally- 
mena area, given me by Mr. Rigby, varies from 2°40 to 2°60, although the rock 
is apparently of uniform texture and is iron-stained on its joints only. A red 
bauxite from Brignoles, Var, gives a result as high as 2°63. 
Since the commercial bauxites are a mixed group of materials, from an alu- 
minium iron hydrate, bauxite proper, to aluminous iron ores and bauxitic clays, 
their specific gravity must obviously vary greatly. Their geological history is 
still obscure in the county of Antrim. I have carefully washed the sample from 
the Libbert mine, in the hope of finding crystals or residues from the rhyolites ; 
but the only reward has been the discovery of decayed and pinkish flints. ‘These 
small fragments can be cut with a knife, like those in some of the Surrey gravel- 
pits; but their form and structure hint at their true nature. I prepared sections 
of two of them, which confirmed the idea that they were truly flint; and Mr. A. 
Vaughan Jennings, to whom the unnamed sections were submitted, referred them 
to the same substance. These flint fragments are characteristically reddened, 
and hence a portion of the altered gravel must have become exposed by the 
denudation of the Lower Basalts before the deposition of the bauxite. It seems 
possible that the latter rock, in its slightly ferruginous varieties, is derived from 
the direct accumulation of fine rhyolitic ash—or, as Mr. Symes suggested, from 
voleanic mud-flows, which carried the material from the flanks of the cones 
into the lowlands.* 
The complete disintegration of the lower surface of the rhyolite at Temple- 
patrick points in the same direction as the rock of Scolboa, which may be called, 
in old-fashioned style, a ‘ claystone-rhyolite ;’ and further deposits of bauxite 
may well lie concealed in the hollows round the massif of Tardree. Against this 
must be set the chemical difficulty of the production of aluminium hydrate, rather 
than kaolin, in rocks where the latter is the ordinary decomposition-product ; but 
it may be remembered that a compound of aluminium is precipitated from any 
aluminium salts which may be in solution by the action of carbonates of the 
alkalies, and that these carbonates are extensively produced during the decomposi- 
tion of granites, rhyolites, and kindred rocks. 
The nature of the precipitate artificially obtained by this reaction is still 
obscure. Formerly it was supposed to be a simple hydrate ; but Muspratt,f using 
a solution of alum and ammonium carbonate, assigned it the formula 3 A1,O,. 
2CO, + 16H,O. Jas. Barratt,t working in Muspratt’s laboratory, and using 
aluminium chloride and sodium carbonate, declared that the product, finally 
washed and dried, was ‘hydrate of alumina perfectly free from carbonic acid.” 
* See Memoir to Sheet 20, Geol. Surv. Ireland (1886), pp. 12 and 16. Analyses on p. 28. 
+ ‘‘On the Carbonate of Alumina,” Quart, Journ. Chem. Soc. London, yol. ii. (1850), p. 216. 
t Ibid., vol. xiii. (1861), p. 190. 
