470 Notices of Memoirs—Dr. H. C. Sorby’s Address. 
or whether greater pressure, and the necessarily slower rate of cooling, were not the: 
more active causes, and the presence of water in one state or another was merely the 
result of the same cause. ‘This is a question which ought to be solved by experiment ; 
but I fear it would be almost impossible to perform the necessary operations in a 
satisfactory manner. ; ; 
What I now propose to do is to describe a particular class of facts which have 
lately attracted my attention, and to show that the crystalline minerals in products 
known to have been formed by the action of heat alone, have a certain very well- 
marked and characteristic structure, which is gradually modified as we pass through 
modern and more ancient yoleanic to plutonic rocks, in such a manner as to show at 
once that they are intimately related, and yet differ in such characteristic particulars 
that I think other agencies than mere heat must have had great influence in produc- 
ing the final result. 
‘In dealing with this subject, I propose, in the first place, to describe the character- 
istie structure of products formed artificially under perfectly well-known conditions, 
and then to pass gradually to that of rocks whose origin must be inferred, and cannot 
be said to have been completely proved. 
Crystalline Blowpipe Beads. —Some years ago I devoted a considerable amount of 
time to the preparation and study of crystalline blowpipe beads, my aim being to 
discover simple and satisfactory means for identifying small quantities of different 
earths and metallic oxides, when mixed with others ; and I never supposed that such 
small objects would throw any light on the structure and origin of vast masses of 
natural rock. The manner in which I prepared them was as follows: A small bead 
of borax was so saturated with the substance under examination at a high temperature, 
that it became opaque either on cooling or when slowly re-heated. It was again 
fused so as to be quite transparent, and then very slowly cooled over the flame. If 
properly managed, the excess of material held m solution at a high temperature 
slowly crystallised out, the form and character of the crystals depending on the 
nature of the substance and on the presence of other substances added to the bead as 
test reagents. By this means I proved that in a few exceptional cases small simple 
solid crystals are formed. More frequently they are compound, or occur as minute 
needles, but the most characteristic peculiarity is the development of complex skeleton 
crystals of extreme beauty, built up of minute attached prisms, so as to give rise to 
what would be a well-developed crystal with definite external planes, if the inter- 
spaces were all filled up. In many cases the fibres of these skeletons are parallel to 
three different axes perpendicular to one another, and it might be supposed that the 
entire skeleton was due to the growth of small needle-shaped crystals all uniformly 
elongated in the line of one crystalline axis, so that the resulting mass would be 
optically and crystographically complex ; but in some cases the different systems of 
fibres or needles are inclined obliquely, and then the optical characters enable us to 
prove that the separate prisms are not similar to one another, but developed along 
different crystalline planes, so as to build up one definite crystal, mechanically com- 
plex, but optically and crystographically simple, or merely twinned. In a few special 
cases there%is a well-pronounced departure from this rule, and truly compound groups 
of prisms are formed. In the centre, that is a definite simple prism; but instead 
ot this growing continuously in the same manner, so as to produce a larger prism, its 
ends, as it were, break up into several smaller prisms, slightly inclined to the axis of 
the first; and these secondary prisms, in like manner, break up into still smaller, so 
as ultimately to give rise to a curious complex brush-like growth, showing in all 
paeens a sort of fan-shaped structure, mechanically, optically, and crystographically 
complex. 
I have done my best to describe these various kinds of crystals seen in blowpipe 
beads as clearly as can be done without occupying too much time, but feel that it is 
impossible to make the subject as simple as it really is without numerous illustrations. 
However, for the purpose now in view, it will, I trust, suffice to have established the 
fact that we may divide the crystals im blowpipe beads into the following groups, 
neh on the whole are sufficiently distinct, though they necessarily pass one into the 
ober. 
1. Simple crystals 3. Fan-shaped compound groups. 
2. Minute detached needles. 4, Feathery skeleton crystals. 
It must not be supposed that crystals of one or other of these groups occur pro- 
