208 E. B. Tawney—Woodwardian Laboratory Notes. 
these full and carefully made collections have acquired fresh interest, 
while at the same time they required some further study. For 
arrangement purposes, and to adapt their names to the modern 
classification, it became desirable to slice and investigate some of 
them microscopically. 
As the collections are due to travels also in Cornwall, the Lakes, 
Cheviots, etc., as well as N. Wales, there is an abundant store for 
future investigation. Choice was made, however, of N. Wales to 
begin with, as only due to the memory of the founder of the 
Cambrian System, and it is convenient to take first that N.W. less- 
explored part of it known as the Lleyn Promontory. 
Prof. Bonney was good enough to make the first selections for 
slicing, and he distributed them among a few Cambridge students 
for investigation.’ 
Among the first so treated was one which at once attracted Prof. 
Bonney’s attention and of which he furnishes the following descrip- 
tion: [P. 85] 
Tn this slide olivine, usually in rather rounded grains, is abundant. 
It is traversed by strings of serpentine, and is sometimes to a 
great extent converted into that mineral, with the usual grains and 
clotted granules of magnetite. Hornblende is also plentiful; it is 
strongly dichroic, changing from pale yellow-brown to rich clove- 
brown as the polarizer is rotated. There is some augite nearly 
colourless in thin sections; also a pale-greenish mineral, which 
occurs in radiated nests, tufted groups of fibres, and associated 
plates, like a mica. This last mineral is strongly dichroic, changing 
from a pale dull green toa pale golden-brown, and with ordinary 
light shows similar but fainter tints, the former when regarded 
perpendicular to the latter when parallel with the principal cleavage 
planes. Associated with it are specks, rods, and clotted bands of 
opacite. It is probably in part undecomposed brown mica, in part 
a species of chlorite (? ripidclite). The latter appears to be of 
secondary origin, and to result in part from the alteration of augite 
or diallage. The grains of olivine are frequently inclosed in the 
hornblende and sometimes in the augite.—T. G. B. 
To a knowledge of the nature of the rock, it became desirable to 
add something of its occurrence, since the Geological Survey Map 
shows Penarfynydd as the sea termination of a long mass of green- 
stone extending N. and §. for some four miles—a mass which has 
been lately touched upon by Dr. Hicks [Q.J.G.8. vol. xxxv. p. 300] 
as containing gabbros intrusive in his Dimetian as well as in Silurian 
(Hicks). 
Accordingly, last summer I made an excursion from Pwllheli, 
stopping a couple of days at Aberdaron for the sake of investigating 
the matter, (and also of searching for a particular rock in the 
Sedgwick Collection presumed to be from the Aberdaron Promontory, 
the extreme end of Lleyn). 
Examining that end of the mass which extends from Plas Rhiw 
1 The initials of contributors will be appended to their communications. 
