364 PROF. J. ^V. JIJDD ON THE PROPYLITES 



altered on the basal plane has been mistaken for true diallage with 

 schillerization on the orthopinacoid. 



The rocks called by the French geologists labradorites, and which 

 1 propose to term " labradorite-andesites," consist of a base which 

 is, in all essential respects, identical with that of the last-described 

 rocks ; but they contain numerous and sometimes very large por- 

 phyritic crystals of labradorite, or of a felspar which is intermediate 

 between labradorite and anorthite. 



Good types of such rocks may be found at Dun-da- Ghaoith (Dun- 

 da-Gu) in Mull, around the southern flanks of Glamaig in Skye, 

 and at Beinn Suardil in the same island. They agree in every 

 respect with the Icelandic varieties so clearly described by Breon. 

 The general features of these rocks, and the evidence they afford of 

 the growth of the felspar crystals subsequently to the consolidation 

 of the rock, have been discussed in a previous communication to this 

 Society *. 



In their altered condition these labradorite-andesites present the 

 most complete analogy with the labradorite-porphyrites, such as the 

 Verde antique of Greece, and the Lambay-Island porphyrite so well 

 investigated by Yon Lasaulx. 



Occasionally rocks of the " trachytoid " type are found passing 

 locally into a perfect glass. An exatnple of this was discovered 

 some years ago by the late Mr. Grieve, and I am indebted to my 

 friend Professor Bonney for calling my attention to it. The locality 

 where this is found is at Bealach a' Mhaim, at the head of Glen 

 Brittle in Skye. The mode of occurrence of this glass is somewhat 

 obscure, but it appears that the glass does not exist like the tachy- 

 lyte-selvages to basalt-dykes, but as local patches in the midst of 

 the andesite. Probably in this, as in cases to be more fully de- 

 scribed in a later portion of this paper, a separation has occurred 

 between the glassy and the crystalline portions of the andesite. 

 The glass has a specific gravitj^ of 2*63, while that of the labradorite- 

 andesite in which it is enclosed is 2*89. 



By the kindness of Prof. Thorpe I have been supplied with 

 analyses of the glassy portions of this rock made in the Chemical 

 Laboratory of the K'ormal School of Science and Boyal School of 

 Mines. 



I. II. III. 



Silica ol-51 62-10 61-80 



Alumina 14-83 15-00 14-91 



Perric oxide 8-41 8-14 8-27 



Lime 3-58 3-07 3-33 



Magnesia 0-28 0-27 0-27 



Soda 6-58 6-42 6-50 



Potash 5-17 5-21 5-19 



Loss on io-nition . , 0-88 0-86 0-87 



101-24 101-07 101-14 



* Quart. Journ. Geol. Soc. vol. xlv. (1889) pp. 175-186. 



