204 
POPULAR SCIENCE REYIEW. 
found that they were only vitrified half through. It was evident, therefore, 
that the forts had been vitrified by means of the wood found imbedded 
among the stones, and that it had also been applied along the inner boundary 
only. 
Absence of Glacial Action in Alaska. — Mr. Dali, who has been recently 
exploring the Alaska district, states that no traces of Glacial action are any- 
where to be found. This opens up several important problems. 
The Structure of Rocks. — The Royal Irish Academy has voted the sum of 
261. to Professor William King, for the purpose of making investigation into 
the jointing cleavage and foliation of rocks. 
Mr. Ruskin on Agates. — Mr. Ruskin continues his curious papers on 
Agates in the Geological Magazine. Some of the plates — which, by the way, 
are tkejbest part of the communication — are very beautifully executed. 
Depth of the Carboniferous Sea. — It would seem from a paper by Mr. 
Somervail, in the Proceedings of the Geological Society of Edinburgh, that the 
minimum depth of this sea was 50 fathoms and the maximum 1,000. 
Mathematical Geology. — The Rev. O. Fisher recently published a very 
mathematical paper on the elevation of mountains by lateral pressure. The 
paper is ably criticised by Mr. David Forbes, F.R.S. ( Geological Magazine , Jan- 
uary), who thus sums up his opinion of it : — “ Mr. Fisher’s paper is a purely 
mathematical one, and altogether beyond the scope of my present communi- 
cation ; yet what I have already stated is but another proof of the tendency 
which mathematicians apparently have to treat experimental data in their 
own way, overlooking the vital importance of thoroughly sifting their 
evidence, or premises on which they base their elaborate reasonings. The 
recent notorious case of the Pascal forgeries is but an instance in point ; 
and a representative of the Press pleasantly remarked that nobody but a 
mathematician could have been deceived by such imperfect evidence.” 
Slates. Felsites — Elvanites. — In the Philosophical Magazine to? January Mr. 
J. S. Phillips points out the use of the microscope in investigating the 
structure of rocks. He describes certain of the above-named rocks from 
Knockmahon, Co. Waterford, and he also gives the analysis of them. He 
supplies, too, an account of the microscopic construction. We quote his 
account of their sections : — 
“ Felsite, spec. grav.=2'64. — Under the microscope this was found to con- 
sist of a colourless and generally amorphous matrix enclosing a few dodeca- 
hedral crystals of quartz and some small crystals of felspar. Other portions 
of the matrix appear to be indistinctly crystalline, and to enclose a few 
lamina; of a greenish mineral, probably chlorite. 
“ Elvanite, spec. grav.=26G. — Examined under a £-inch objective, this 
rock is seen to be composed of an amorphous greyish matrix, in which are 
porphyritically imbedded crystals of quartz and felspar, the latter being 
chiefly oligoclase. In addition to these, a few small crystals of some horn- 
blendic mineral are sparingly disseminated throughout the mass. It was 
further observed that the larger quartz crystals are sometimes penetrated by 
crystal* both of fel-par and hornblende ; and when examined under a high 
power, the quartz is seen to contain fluid-cavities. 
u Metamorphosed Slate?, spec. grav. = 2*65. — A section was found under 
the microscope to be chiefly composed of felspathic-looking crystals crossing 
