
TOURMALINE, &c., WITHIN MICA AND OTHER MINERALS. 551 
in a very irregular manner from a nucleus, sometimes having the form of a thin 
film; sometimes of oriental characters ; and sometimes it is disseminated in grains 
so extremely minute, that the flame of a candle seen through it is surrounded with 
a halo of five or six perfectly-formed coloured rings. 
3. Distribution of Quartz in Mica. 
In mica from various localities, I have found large crystallisations of quartz, the 
quartz replacing the mica. I have never even once met with a regular crystal of 
quartz; and what is curious, all the crystalline masses of it which I have exa- 
mined have their axis of double refraction in the plane of the laminz of mica. 
Tn some very large specimens of Bengal mica given to me by Mr Swinton, I have 
found layers of quartz, several inches in area, and about the 200th of an inch 
thick. The two surfaces of the plates are exceedingly inequal and corrugated, 
owing to the circumstances under which they were formed, but they possessed 
regular double refraction, and gave the colours of polarised light. 
4. Distribution of Titanium in Amethyst. 
While examining, many years ago, along with the late Marquis of Norra- 
AMPTON, several bags of amethyst which had been imported into Scotland from 
the Brazils, we were surprised to observe a number of fine pyramidal crystals, 
which seemed to have a powdery matter distributed through their mass. Upon 
more narrowly examining these crystals, I found that this dust formed an inner 
pyramid, all the faces of which were parallel to the faces of the pyramid of ame- 
thyst. When two parallel faces were ground upon the pyramid, and perpendi- 
cular to its axis, the particles of dust. were seen by the microscope to consist each 
of several spicular crystals of titanium, crossing one another at angles of 60° and 
30°, and forming distinct groups. In one crystal there were two interior pyra- 
mids composed of these groups; and it will be seen, from the explanation which 
I shall presently give of this phenomenon, that there may be any number of such 
pyramids. 
As the crystals of amethyst are supposed to have been produced by the gradual 
enlargement of a small crystal placed in an amethystine solution, we have only to 
assume that a solution containing titanium has been introduced into the ame- 
thystine solution at different times during the growth of the crystal. The small 
crystals of titanium will deposit themselves on each of the surfaces of the pyra- 
mid; and when the whole of the introduced titanium has been thus deposited, 
the enlargement of the amethyst will go on, leaving a pyramid of titanium crys- 
tals in its interior. Ifa second solution of titanium is introduced, a second pyra- 
mid of its particles will be formed in the same manner; and this process may 
be repeated any number of times. 
If we now suppose that the amethystine solution is exhausted, just when the 
VOL. XX. PART IV. 7K 
