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MESSRS. G. A. J. COLE ATfD J. W. GREGORY 



crags forming the east side of the Chenaillet valley, south of Mt. 

 La Plane, the spheroids appear piled regularly one above the other, 

 forming walls as it were, which are divided by conspicuous vertical 

 joints ; but the structure is more commonly irregular, the masses 

 resembling pillows or soft cushions pressed upon and against one 

 another, each cliif-face thus exhibiting a number of swelling sur- 

 faces and curving lines of junction (fig. 4). Examined more 

 closely, small vesicles are seen in these rude spheroids, especially 

 towards the margins ; and in some places, as on the east of Mt. La 

 Plane, the whole rock becomes vesicular and slaggy. The surfaces 

 of the masses are covered by a crust of variolite, from 1 to 7 or 8 

 centim. thick. The spherulites or " varioles " are grouped or drawn 

 out in bands parallel to the surface, being in some places almost 

 microscopic, in others 5 centim. in diameter. 



The coarsest variolite that we are acquainted with occurs on a 

 little plateau above the pine-woods on the north end of the ridge of 

 Le Chenaillet. The ground here, with the large sj^herules pro- 

 jecting from the weathered surface, reminds one of the pyromeride- 

 area of Digoed near Penmachno in JN'orth Wales. 



Fig. 5. — VarioUte-diahase ; north end of Le Chenaillet Ridge. 



A and C. Compact diabase, 

 B. Variolite, 5 centim. thick. 



D. Variolite, bent and infolded, 3-4 centim. thick. 



E. Compact spherulitic diabase. 



This, then, is the typical mode of occurrence of the famous Vario- 

 lite of the Durance. It is (everywhere a selvage to compact and 

 ordinary basic igneous masses, just as tachylyte so frequently bor- 

 ders the intrusive basalts of our Western Isles of Scotland*. But in 



* See P. R Kendall, 

 Mag. 1888, p. 555. 



On some Occurrences of Tachylyte in Mull," Geol. 



