ANT) SPHEROIDAL STRUCTURE.
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the mass. The outer part of the mass of basalt at the Spindle and Montiquey (mentioned above) exfoliates in rather thin plates ; and the columns are, so to speak, enclosed in a fissile spheroidal shell a few inches thick.
The phenomenon is very conspicuous in some of the trachytic rocks of the Auvergne ; those in the Cantal and Mont Dore are so fissile as to be used for roofing-slates. The well-known Roche Sanadoire and lloche Tuilliere*, the latter of which is extensively quarried for the above purpose, are very remarkable instances of this structure, and throw much light upon its origin. I shall therefore describe them in some little detail. "Near the highest part of the great Auvergne plateau, from which rise the groups of the Puy-de- Dome volcanoes and the chain of the Pic de Sancy, and at the northern extremity of the latter, two rocky bluffs rise on either side of the deep cirque-like head of a wooded glen. That on the right bank of the valley is the Sanadoire, on the left La Tuilliere. Both are phonolite; but the character of the rockf, and still more its structure, are different.
The Roche Sanadoire forms the extremity of a rather short spur from the Puy de T Angle. On the southern side it is not very con- spicuously columnar; but the rock, which is exposed in several abrupt crags, is divided into a vast number of small rather irregular prisms, with numerous cross joints. Yhen, however, a view is gained of the precipitous western face, the columnar structure becomes more conspicuous, together with another set of divisional planes, which give a more fissile character to the rock. A reference
Fig. 2. — Columnar and Fissile structures. Roche Sanadoire.
A, C. Jointed columns. B. Fissile bands.
- Mr. R. Mallet (Phil. Mag. ser. 4, vol. 1. p. 218) calls the rock of La
Tuilliere basalt, and the fissile structure a " slaty cleavage" due to subsequent pressure. The rock is a phonolite, and has not the slightest resemblance to basalt ; and the structure, I trust to show, is not a case of ordinary cleavage.
t For full account see ' Neues Jahrbuch fiir Mineralogie,' 1872, p. 351. Q. J. G. S. No. 126. L
