214 E. B. Tawney—Woodwardian Laboratory Notes. 
Dlanfaelrhys.—It differs sufficiently from the larger mass close by to 
justify the mapping, even if the black shales were not seen in the 
ravine on the W. An apparently fresh piece shows numerous black 
spots like delessite, with smaller ones of magnetite in a sap-green 
coloured ground of rather fine grain. No effervescence with acid. 
Microscope.—Plagioclase seems the most abundant constituent; it 
still preserves its banding, but only shows dull colours; the prisms, 
ete., are arranged as in any dolerite. They are full of very long 
slender microlites with a slight greenish tint, and are like those often 
referred to tremolite. The augite is gathered together in compound 
groups and also scattered throughout in crystals of moderate size; 
they are much attacked by decomposition, but the parts left un- 
affected polarize brightly. 
A large portion of the slide is taken up with viridite areas, some 
of large size with a somewhat scalloped outline, and banding 
arranged parallel thereto (as in an agate); it has no dichroism, and 
is dark indigo under crossed prisms; it exists also in smaller por- 
tions scattered throughout the slide. There is a fair quantity of 
black oxides of iron, some titaniferous changing to leucoxide, some 
forming long linear crystals remain jet black in reflected light and are 
probably magnetite. There is also a good deal of dirty matter about 
the slide, granulated material discoloured by iron oxide; this arises 
no doubt largely from augite, but may partly also be the remains of 
a glass which has disappeared. 
This mass is intruded into shales which, as noted above, must be 
Tremadoc or Arenig. I was unsuccessful in finding fossils in the 
immediate neighbourhood. 
Diabase, quarry at top of Aberdaron village.—A dark green rock 
with narrow needle prisms of felspar; no effervescence with acid. 
The ‘rock is very much like the next one from Penycil except that 
delessite spots are here more abundant and larger. Pyrites is seen 
in the hand-specimen, which is of medium grain. 
In the Sedgwick collection is one [P. 34] which is from near 
Aberdaron, five miles to the north, being labelled “hill close to My. 
Thomas’ house,” Treferaig farm: it is somewhat similar to these 
bosses (which are only distant on the map about 14 miles) —its 
augite a little larger. As its felspars are much decomposed, I chose 
instead the specimen obtained fresh from the interior of Aberdaron 
quarry for slicing. 
Microscope.—The plagioclases form the bulk of the slide arranged 
in the form of long prisms ; they retain a certain amount of freshness, 
as evidenced by brightly polarizing qualities; they contain in great 
abundance pale greenish microlite needles set in various directions. 
The augite is in a minority, the crystals small, of corroded outline, 
scattered through the slide, and seem mere remnants. 
Viridite tracts occupy a large proportion of space ; they are slightly 
dichroic, showing light green and pale salmon colour, of radiating 
fibrous structure, which show aggregate polarization of bright tints ; 
smaller patches are scattered through the slide, and it permeates 
everywhere. Abundance of opaque ferric matter, particularly round 
the augites. 
