202 Prof. T. G. Bonney—Pebbles in the Bunter Beds. 
filmy greenish mineral associated with granules of hematite. The 
former proves to be an aggregate of a doubly refracting mineral, 
which is probably a hydrous magnesia-iron silicate, akin to serpen- 
tine; the whole being, I have no doubt, a secondary product after 
a magnesia-iron mica. 
The third specimen exhibits macroscopically a very marked fluidal 
structure, the ground-mass being crypto-crystalline, with some ap- 
proach to a spherulitic arrangement, among which are patches of 
interstitial quartz, and of a pinite-like mineral which may replace 
a felspar. This and the first one have no doubt once been very 
typical rhyolites. 
Two rather different rocks remain to be noticed—one a pale pinkish 
microgranular specimen with numerous dark spots, sometimes 
nearly a quarter of an inch in diameter, with irregular edges, looking 
somewhat as if a greenish ink had been dropped on the stone and 
allowed to dry. Microscopically examined, the rock has a minutely 
micro-crystalline, almost crypto-crystalline ground-mass, of quartz, 
and of felspar more or less decomposed, intermingled with specks of 
(probably) tourmaline. The larger dark spots are seen to consist of 
aggregate granules and crystallites of tourmaline, generally olive- 
brown, but occasionally indigo-blue, associated in the case of the 
larger patches with small crystals of quartz. Many of the larger 
among these are so irregular in form that it is a matter of conjecture 
what was the original mineral, but among the smaller are many 
single crystals, which exactly recall the outline of a crystal of 
biotite cut transversely to the basal plane. JI have therefore no 
doubt that the tourmaline in the main has replaced magnesia-mica. 
The external boundary of the elongated crystals just described is 
irregular, like that of a mica plate sketched by a tremulous hand. 
This I explain by the fact that tourmaline can be formed from felspar 
as well as from mica, and so the new mineral would occasionally 
encroach upon the ground-mass. We accordingly have here a tour- 
malinized mica-felsite ; for the mica is not abundant enough to allow 
us to call it a minette. 
The last specimen has a very dark ground-mass, in which are 
scattered pale pinkish crystals of felspar, of elongated outline, some 
of which are nearly an inch in diameter; these are seen to be 
speckled with a blackish mineral. The ground-mass is minutely 
micro-crystalline, consisting of irregularly crystallized very decom- 
posed felspar (the chief constituent) with quartz and tourmaline. In 
this are scattered tourmaline, both in tufts and apparently replacing 
mica: grains of quartz of various sizes up to about 0-1 inch (as 
described above), and crystals of felspar. The largest felspar crystal 
lies in the slide; it is much decomposed, and incloses aggregates 
which seem to be a mixture of tourmaline and opacite, and so 
have probably replaced some other mineral. It is a twin apparently 
of the Carlsbad type, and appears to be orthoclase; if this, as is 
very possible, be the dominant mineral, it must have had the tabular 
habit of sanidine. The rock then may once have been a rather 
micaceous and markedly porpbyritic quartz-felsite. 
