af 
E. B. Tawney—Woodwardian Laboratory Notes. 213 
bisilicates ; it is much attacked by alterations, and often quite an 
opaque granulated substance; the crystals are frequently crossed by 
eracks of green chloritoid matter; sometimes they are partly 
replaced by quartz. In some tracts, where traversed by quite a 
network of the green veins, they appear very fresh, polarizing 
brightly. Titaniferous iron is abundant. - 
I regret not having any of the foreign hornblendic gabbros for 
comparison, such as have been described from Le Prese, or protero- 
? 
bases, such as have been described from the Fichtetgebirge, and by 
Dr. Stache from the Zwélfer-Spitz district (Jahrb. k. k. geol. 
Reichsanstalt, 1877, vol. xxvii. pp. 207-216). To Prof. Bonney I am 
indebted for the loan of thin slices of Cornish gabbros showing the 
paramorphic origin of hornblende. As gathered from his description 
too, they lead me to consider the present case as of quite a different 
kind. 
Diabase of Mynydd y Graig, ete.—Of the diabase, a piece was 
selected from the quarry beyond Plas Rhiw as likely to be fresher, 
and it seems the same type of rock all down to Penarfynydd Pro- 
montory. The crystals are of medium grain, no large ones among 
them ; a brownish-grey rock speckled with much white, the bronze- 
coloured pyroxene and felspar about equal : the white part effervesces 
abundantly with acid, and is sometimes discoloured to a pale-green. 
Microscope.—The bulk of the rock consists of augite of pale- 
yellowish tint in thin sections, which polarizes vividly ; most of the 
crystals are developed at their margins into or sometimes penetrated 
by dichroic brown hornblende, not quite so strongly dichroic or so 
well pronounced as in the last rock, and of rather dull doubly refrac- 
tive power compared to the augite. As not unusual in such cases, 
the two intergrown crystals are orientated in the same manner, and 
in one compound crystal we obtained a maximum extinction angle of 
06° for the augite, and about 16° less for the hornblendic portion. 
One pale crystal behaving like an orthorhombic mineral is supposed 
to be an orthodiagonal section of the augite rather than another 
species. Undergoing change the hornblende becomes full of crowded 
grains of magnetite collected together and surrounded by an irregular 
area of fibrous zeolitic crystals, while in the centre may be a colour- 
less tract of very feeble doubly refractive power, under crossed 
prisms the indigo black showing only slight change during revolu- 
tion of the plate. 
These rocks are noticed in the Survey Memoir (l.c. p. 174) as 
varieties of greenstone, which “further north and west of Mynydd 
y rhiw passes into syenite.” This is a view entirely different from 
that of Dr. Hicks mentioned above. 
The next two rocks are also recognized as intrusive (7b. p. 174-8). 
The slates which are described as dipping under the greenstone of 
Pen y cil are there ascribed to the Bala series with some doubt. I 
was unsuccessful in the search for fossils in the black slates in which 
the Aberdaron mass is intrusive, but they may turn out possibly to be 
Lower Bala (Sedgwick). 
Diabase from greenstone patch on map below Tynyrhedyn 8S. of 
