146 
THE GEOLOGIST. 
very same portion of this mineral. One is the chalcedonie, or apparently 
non-crystalline. The lecturer used the word apparently, because the sub- 
stance has been found to be composed of minute crystals. Close in con- 
tact with this is the other state, which is more or less non-crystalline; and 
then occurs a sort of granite deposit, which, seen under a magnifying glass 
of high power, is seen to consist of distinctly crystalline quartz. Amor- 
phous silica exists in the shells of living infusoria ; and it is found also in 
a siliceous concretion met with in the joints of the bamboo, known as 
tabasheer ; it occurs also in petrified wood. 
The lecturer next took up the subject of flint — a subject which has re- 
cently given rise to so much discussion. All were acquainted with the 
discovery of those wonderful nodules of flint in chalk, the formation of 
which is involved in doubt, notwithstanding that they are associated with 
the presence of sponge, and so on. There are, however, chemical difficul- 
ties which arise, and which will require much time to solve completely. 
When we take one of these dark flints, and expose it gradually to a tem- 
perature which we bring at last to a bright redness, it becomes very tender 
and fragile, and it would become especially fragile and perfectly opaque if 
heat were rapidly applied. It is in this ^A ay that the flints prepared for 
the use of potters are rendered fragile in the first instance, preparatory to 
grinding. Now when we examine s])ecimens of flints we are frequently 
struck with an opaque white outer film, where the flint has lain in contact 
with chalk. This film or rind, if I may so call it, extends to a considerable 
depth. Well, what is it? Is there any chemical difference between this 
white rind and the inner dark portion of unchanged flint? This is a ques- 
tion which we have examined with some care, and by the aid of the most 
careful chemical analysis we are unable to detect any sensible difference 
between the two. The only conclusion therefore is, that the difference of 
appearance is owing to some molecular change ; but how it is excited the 
lecturer was not prepared to say — whether by the slow operation of time, 
or M hether, perhaps, by simple vibrations. It has often struck me, with 
regard to the flints which I have seen on the seashore, which have pre- 
sented a white appearance, that possibly that appearance may have been 
caused by some operation of that kind ; but upon that point he spoke with 
the greatest possible reserve. It is of very great importance to know 
exactly the effect of weathering upon the surface of flints, especially with 
regard to the great question now agitating the public mind concerning 
primeval man. Here is one of those wonderful things hewn out of flint 
by our rude forefathers, for the loan of which he was indebted to his friend 
Dr. Falconer. It presents a fracture at one end, which is very different 
from the contiguous portion ; but at another portion there \a as observed 
to be the characteristic dark appearance of these ordinary flints. He had 
been examining this specimen, and upon the whole he thought it must 
have been long exposed, subject to some condition by which the surface 
has been changed. Then comes the question, whether it is possible to 
imitate these signs of age upon counterfeits of ancient flint implements. 
He thought a few experiments would very soon settle the point whether 
it is possible to give a surface very much resembling that which we find in 
these old weapons, by some chemical means. 
With regard to the permeability of these flints by liquids, it is astonish- 
ing how pervious they are. If we immerse flint in sulphate of indigo and 
water, and keep it there some weeks, the indigo will penetrate to some 
depth. 
It has been alleged, and it is a point of some importance, that silica may 
