1886.] Alteration by Heat in certain Vitreous Rocks. 431 



The following examples which have been operated upon are few, 

 but typical, and the alterations which they have undergone will be 

 found to have a certain petrological significance. 



The first subject taken for experiment was a small fragment of the 

 well-known pitchstoue of Corriegills in the Isle of Arran. This was 

 kept at a temperature ranging from 500° up to about 1100° C, during 

 a period of 216 hours. The change visible at the end of the nine days' 

 heating is not so strongly marked as might have been expected, the 

 fragment still exhibiting a resinous lustre, but the colour, originally 

 a deep green, has been altered to a deep reddish-brown or chocolate 

 colour. The rock in its normal condition has already been described 

 by Mr. S. Allport* and others, and a section cut from the specimen 

 before heating presents exactly the same characters shown in Airport's 

 drawings, published in the " Geological Magazine," in Vogelsang's 

 " Krystalliten," Plate 13, fig. 2, and in Zirkel's " Mikroskopische 

 BescbafTenheit der Mineralien und Gesteine," fig. 43. 



A section made from the specimen now described, but prepared 

 prior to heating, has furnished the figs. 1, 3, and 5, on Plate 3. The 

 three right-hand figures on the same plate have been drawn from a 

 section made after nine days' heating at a temperature ranging from 

 500° up to about 1100° C. For the sake of comparison the figures on 

 opposite sides of the plate have been drawn under the same amplifica- 

 tion. 



When the artificially altered rock is examined under a power of 

 25 linear it presents the general appearance shown in fig. 2, Plate 3. 

 On comparing this with fig. 1 on the same plate we see that a marked 

 change has taken place. The clear spaces surrounding the crystallites 

 or belonites of hornblende have increased, while the dusty-looking 

 matter no longer shades off into the clear glass but lies within more 

 or less sharply defined boundaries. It also presents a coarser appear- 

 ance than in the section taken from the unaltered specimen. With 

 increased amplification the character of the change becomes more 

 clearly perceptible. 



In fig. 4, Plate 3, we find, on comparing it with fig. 2 on the same 

 plate, that the hornblende belonites themselves have undergone very 

 considerable alteration. They have to a great extent lost their frond- 

 like appearance. Their delicate lateral growths seem to have shri- 

 velled up, and their green or greenish-brown colour has changed to a 

 deep rusty brown. Their stems or central rods have become opaque, 

 and the lateral fringes frequently share this opacity. They seem, in 

 fact, mere withered representatives of the greenish fern-like crystallites 

 which occur in the natural state of the rock, and the change appears 



* "On the Microscopical Structure of the Pitchstone of Arran," " Geol. Mag./' 

 1S72. 



