ANT) SPHEROIDAL STRUCTURE. 



143 



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 



