ON THE CRYSTALLINE ROCKS OF ANGLESEY. 320 
A number of qualitative determinations have been made also. Most 
interesting among these are three of barytes from veins in Bodafon 
mountain and in the carboniferous limestone; one of graphite in a 
peculiar rock from Llanwenilwyfo ; and one of malachite in a mica schist 
near Valley. Copper pyrites were also determined in a vein-stone in 
Ordovician shales near Rhosgoch. 
Mr. Hughes contributes a report on the methods used, particularly 
on the precautions that have been taken to exclude error, and adds 
comments on a number of the analyses. 
The Committee ask to be reappointed, and to retain the small balance 
left from the grant. 
APPENDIX. 
Methods of Rock Analysis. By Joun Owen Hucuss, B.Sc. 
The weight of rock required for a complete analysis, adequately repre: 
senting the average of the rock-mass, varies with the texture and homo- 
geneity of the particular rock. 
For the first four or five analyses samples weighing only about five 
or six grams were available ; these, however, were very homogeneous in 
character, and the accuracy of the analyses would not suffer greatly from 
this cause. In the remaining rocks it was thought advisable to use larger 
quantities, thirty to fifty grams being the usual ‘grind’ prepared. In 
this way, after thorough mixing, it is possible to obtain a sample which 
better represents the whole rock-mass than if a smaller weight had been 
taken. specially is this the case where there is banding or veining. 
In all cases the outside or weathered surface of the rock was rejected, 
the fresh cores being alone used. 
Preparation of the Sample.—The time and the labour involved in the 
process of grinding the rock are great, and it is desirable that these should 
be reduced as much as possible. The following different methods have 
been employed :— 
(1) The rock was ground up in a ‘diamond’ steel mortar in small 
quantities at a time, and the resultant mixture sifted through a ninety- 
mesh wire sieve. The part which did not pass through was once more 
crushed in the mortar, and again sifted, the process being repeated until 
all the rock had been in this way reduced to powder. 
(2) The rock was broken into small pieces by means of a hardened 
steel hammer on a hardened stee] plate. These were then crushed in the 
steel mortar and reduced to powder as in (1). 
(3)! In this method the rock was broken up ina special steel revolving 
crusher (sold by Becker & Co.), and reduced directly to a state of fine 
powder, most of which passed directly through the ninety-mesh sieve. 
The part remaining when passed again through the crusher once or twice 
was pulverised completely, any hard particles being finally ground up by 
hand in an agate mortar. 
This last method might be open to one objection—viz., contamination 
of the rock powder by metallic iron from the crusher ; but this being of 
‘ This method could only be employed for two rocks, as the crusher was not 
bought until March. 
Y2 
