174 AGATES, CARNELIANS, AND JASPERS. 
of studying these facts for themselves, to pay a few visits to 
the Gallery devoted to Scottish Geology and Mineralogy in 
the Edinburgh Museum of Science and Art, where they will 
find the best collection of Scottish Agates in existence, made 
by Dr Heddle and Mr R. Miln, presented to the Museum 
by Mr Thoms, and arranged by the present writer, who has 
written a guide for the use of visitors who come to study 
the collection referred to. 
APPENDIX, appED SEPTEMBER, 1900. 
In Vol. I. of the Popular Science Review, pp. 23-28, is an 
instructive article by Mr Rudler, entitled “ Agates and Agate 
Working,” from which the following notes are taken :— 
(1) Sir David Brewster (Phil. Mag. (3), vol. xxii. p. 213) 
measured the thickness of some of the layers of an agate, 
and found that they ranged from 1/17220 of an inch to 
1/55760. (2) It was the late Professor Haidinger who sug- 
gested that the materials of an agate did not come in through 
the so-called tube of entry, but by a general exudation through - 
the walls of the cavity. (3) Bischoff (Lehrbuch d. Chem. 
Geol., vol. iii. p. 636) estimates that the deposition of a layer 
of agate one line in thickness requires twenty-one years. 
In order to form one pound of amethyst, at least 10,000 lbs. 
of water must have been introduced into the cavity and 
evaporated ; an action which has been estimated to occupy 
the vast period of 1,296,000 years. [But agates many 
pounds in weight were certainly formed in the interval of 
time between the cessation of the Caledonian Old Red vol- 
canic action and the commencement of the conditions to 
which the Upper Old Red was due, seeing that large agates 
derived from the older formation occur in the conglomerates 
of the newer—J.G.G.] (4) Regarding the Tube of Escape : 
Herr Von G. Lange, of Idar, in his “Die Halbedelsteine aus 
der Familie der Quartze” (1868), p. 17, seems to have been 
the first to recognise the true nature of these canals, which 
he describes as channels of egress rather than of ingress. 
(5) Mr Rudler’s paper also includes an interesting account 
of the history of the agate industry at Oberstein, to which 
the reader is referred for further details. 
