240 Transactions of the Royal Society of South Africa. 
The material investigated was supplied to the Geological Department 
of the University of Cape Town by Foote of Philadelphia, and is labelled 
as having been obtained from Guanajuato, Mexico. 
The specimen represents a portion of a succession of deposits which pre- 
sumably originally lined a rock cavity. Several " forms " of silica — some 
chalcedonic, others opaline — are represented, and, in our opinion, judging 
from this single specimen, a definite order of deposition is observable. 
We infer that, throughout a period of continuous deposition, the chalce- 
donic varieties were deposited at the commencement, and the opaline 
varieties towards the close, of such a period. 
These varieties, enumerated in the order in which we consider them to 
have been deposited, are as follows : — 
Chalcedonic Silica. 
1. Dark chocolate-brown variety resembling jasper. 
2. Opaque, white ; in parts bluish-white. 
3. Pale blue or bluish-white, semi-translucent ; botryoidal (pale brown 
by transmitted light). 
Opaline Silica. 
4. Clear, colourless, botryoidal hyalite. 
5. Extremely pale bluish- white, opalescent or slightly clouded hyalite ; 
spheroidal in form, and much less common than the colourless and water- 
clear hyalite. (Since this variety generally occurs forming semi-spheroid 
masses on the surface of the clear hyalite, no difficulty is experienced in 
distinguishing between the two — particularly when both are immersed in 
water, — but, when the slightly clouded variety is detached from the clear, 
optical examination proves the surest method for its identification. It is 
important to note also that, in certain parts of the specimen, the small 
semi-spheroids of this variety are coated with a thin layer of clear hyalite.) 
Apparently the forms of silica enumerated above were not the result of 
one continuous period of deposition, but of several such periods ; nor was 
deposition uniform over the whole surface of the cavity throughout each 
period. Several repetitions of the sequence 2, 3, 4, 5 are observable. 
In the following we refer principally to two kinds of thin sections, 
which, for convenience, we propose to name " diametral " and " tangential " 
respectively. By " diametral section " we mean one cut approximately 
through the centre of a spherulite ; by " tangential section " one prepared 
from the surface layer of such a body. 
Throughout the preparation of any one section the material was closely 
observed in order to see whether the grinding — the process by which all 
