360 PEOF. J. W. JUDD ON THE PROPYLITES 



and a little glass, with only a few scattered crystals of ferromag- 

 nesian silicates. These rocks are usually pale-coloured or nearly 

 white and have a low specific gravity (2-55). Other hornblende- 

 andesites, like those of Mhaim Clackaig in Mull and of Glen Brittle 

 in Skye, exhibit a dark green, often nearly black colour, and have 

 a much higher specific gravity (2-7 to 2-8). 



I have not detected any completely glassy forms of these rocks, 

 nor any examples in which primary quartz occurs. Some of the 

 more highly crystalline types, howevei', exhibit the granophyric 

 structure and contain free quartz, which I believe is of secondary 

 origin. 



Hornblende-mica-andesites, with or without pyroxenes, are very 

 abundant in the district. They constitute rocks of a pale grey 

 colour and a more or less fissile character, which form well-marked 

 lava-streams, some of which are found intercalated among the 

 basalts of the plateaux. 



In some of the best-preserved of these rocks, crystals of unmis- 

 takable enstatite (bronzite) make their appearance ; and, as what 

 may be the products of alteration of this mineral are seldom absent 

 in the more altered varieties, it may probably be assumed that the 

 majority of the rocks of this type in the Western Isles of Scotland 

 must be referred to the hornblende- and mica-andesites containing 

 enstatite. 



Some of these rocks exhibit a character lately referred to by 

 Dr. Osann * and by Mr. Teall t. Gas-cavities are found filled with 

 glassy matter that seems to have oozed out of the ground-mass of 

 the rocks into these empty cavities. In the case of some of the 

 Scottish rocks it is curious to find that the glass in these cavities 

 exhibits a markedly banded structure. 



Mica-andesites. — While there are probably some examples of true 

 mica-andesites it must be remembered that a dark brown biotite is 

 among the commonest of the secondary minerals in these propylitic 

 rocks. Good examples of trne mica-andesites occur at certain 

 points at Mull and also in Eigg. These biotite-andesites pass by 

 insensible gradations into the hornblende-andesites ; in some cases 

 the amphibole being the predominating constituent, in others the 

 mica. As a rule among the altered rocks, the hornblendes tend to 

 disappear by passing into chlorites, with or without the separation 

 of magnetite, while the biotite seems to increase in amount, either 

 by the growth of original crystals of the mineral or by the develop- 

 ment of new secondary crystals. 



Diorites. — Of the common or hornblende-diorites we cannot find 

 better examples than those which occur in the deep Corry exposing 

 the central mass of Beinn Talaidh in Mull. From an almost 

 perfectly holocrystalline rock every gradation can be traced, through 

 beautiful granophyric varieties, into the lava constituting the peri- 

 pheral portions of the mass, which is, as we have seen, a typical 

 hornblende-andesite. 



* Zeitsc'hr. d. deutsch. geolog. Gesellsch. xli. (1889) p. 304. 

 t Geol. Mag. dec. iii. vol. vi. (1889) p. 481. 



