Scientific Intelligence—Geology and Mineralogy. 385 
scribed as remains of trilobites.—Keilhaw’s Gaea Norwegica; Part II. 
. 285. : 
rn 4. New Proof of the Cantal being a crater of Soulevement.—The 
Cantal is almost entirely composed of trachyte, and its general aspect 
presents a vast cone, having in its centre a gigantic hollow of about five 
English miles in diameter. Deep valleys diverge from this centre on 
all sides, like the spokes of a wheel, and impart to the whole mountain 
mass a peculiar character, which, combined with various other pheno- 
mena, has induced Messrs Elie de Beaumont and Dufrénoy to consider 
it as a crater of soulevement. The“trachyte of the Cantal generally 
occurs in the form of great nappes, which rise with a gentle slope in 
the direction of the central depression. Its ordinary appearance is that 
of a breccia, whose fragments and basis being of the same nature, can- 
not be distinguished from each other; but, notwithstanding this frag- 
mentary appearance, all its component parts are contemporaneous. The 
name of trachytic tufa has been given to it,—an expression which 
conveys the idea, that the matter issuing from the interior of the earth 
in a pasty state, has given rise, in the yoleanic opening itself, to frag- 
ments which were immediately united together by the flowing mass. 
The nature of the rock is displayed in all the escarpments; but it is 
exhibited with peculiar distinctness in the tunnel of nearly 4000 Eng- 
lish feet in length, which has been pierced between the valleys of 
Aurillac and Murat, on the road from Paris to Montpellier, for the pur- 
pose of rendering the journey less dangerous in winter. This gallery 
has also afforded the means of studying the numerous veins which tra- 
verse the mass of trachytic tufa. The uniformity of the rock through 
which the tunnel of Lioran has been carried, is one of the most interest- 
ing facts revealed by this great work of art. We thus learn that it is 
one and the same nappe of trachyte which is traversed throughout the 
whole length of the tunnel,—a circumstance opposed to the supposition 
adopted by some geologists, that the whole mass of the Cantal has been 
produced by the accumulation of successive eruptions ; for we have in 
this uniformity one of the most certain proofs of its formation by souleve- 
ment.* 
5. On the Cause of the Colours in Precious Opal. By Sir David 
Brewster.—This gem is intersected in all directions with colorific planes, 
exhibiting the most brilliant colours of all kinds. The cause of these 
colours has never, we believe, been carefully studied. Mineralogists, 
indeed, have said that they are the colours of thin plates of air occu- 
pying fissures or cracks in the stone; but this is a mere assumption, dis- 
proved by the fact, that no such fissures have ever been found during the 
processes of cutting out, grinding, and polishing, which the opal under- 
goes in the hand of the lapidary. In submitting to a powerful micro- 
scope specimens of precious opal, and comparing the phenomena with 
those of hydrophanous opal, Sir David Brewster found that the colorific 
planes or patches consist of minute pores or vacuities arranged in paral- 
lel lines, aud that various such planes are placed close to each other, so 
as to occupy a space with three dimensions. These pores sometimes 
* From M. Dufrénoy’s Report on a Memoir by M. Rozet on Auvergne.— 
Comptes Rendus de V Academie des Sciences, vol. xviii. p. 133. 
VOL. XXXVIII. NO. LXXVI.—APRIL 1845. 2B 
