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Augite-porphyrites (altered augite-andesites) occur in Glencoe. One 

 type of rock is reddish in colour and distinctly porphyritic. The porphy- 

 ritic constituents are turbid felspar and augite. The felspars, in the slides 

 examined, show no trace of individual action ; they are almost opaque. 

 The augite occurs as grains rather than as crystals and is often perfectly 

 fresh. It is almost colourless. Green pseudomorphs probably representing 

 a second bisilicate are also present. The ground-mass is so crowded with 

 granules and flecks of opacite, viridite and ferrite that it is impossible to 

 determine its original character. Another type of rock from the same 

 locality, represented by a slide lent to the author by Mr. ALLPORT, differs 

 from the above in having a ground-mass composed of a micro-crystalline 

 aggregate of quartz and turbid felspar. In the character of their ground- 

 masses these rocks differ from the typical andesites and approximate to the 

 felsites. 



Two other slides of holo-crystalline rocks, allied to the above, from 

 the same locality have also been lent to the author by Mr. ALLPORT. One 

 shows turbid plagioclase often containing granules of epidote, greenish 

 idiomorphic hornblende more or less changed to chlorite, grains of iron-ore 

 and a micro- or crypto-crystalline ground-mass (hornblende-porphyrite). The 

 other is not conspicuously porphyritic ; it is composed of large felspars 

 usually giving lath-shaped sections, augite, hornblende, biotite and a small 

 amount of quartz which fills up the spaces between the felspars. This rock 

 might be described as an augite-biotite-diorite. It is probably merely a 

 local modification of the andesitic rocks. 



Holo-crystalline rocks intermediate in texture between the diorites and 

 andesites occur in Glen Etive. The interstitial matter in these is micro- 

 pegmatite. 



Hornblende-biotite-porphyrite with a micro-crystalline ground-mass 

 occurs also at the Falls of Bruar, near Blair Athole. The plagioclase of 

 these rocks is often crowded with scales of white mica. 



A very remarkable and somewhat exceptional rock is found asso- 

 ciated with the quartzite of Canisp in Sutherlandshire. Specimens in 

 the writer's possession show large crystals of plagioclase (oligoclase or 

 oligoclase-albite) in a reddish matrix. Under the microscope augite and 

 biotite may be also detected amongst the minerals of first consolidation. 

 The augite may be colourless or green. Sometimes the green variety forms 

 a zone round the colourless variety, the surface of separation being sharp 

 and well defined. When this is the case the two varieties possess ' slightly 

 different optical properties. Thus, in a section, approximately parallel to 

 the clino-pinacoid, the extinction angle in the colourless kernel was 34, that 

 in the green zone 40. The green variety does not appear to be the result 

 of the alteration of the colourless variety. It is worthy of note that green 

 augites of this type occur in acid rocks. The biotite is generally more or 

 less altered to chlorite. The ground-mass is a micro-crystalline aggregate 

 of quartz and turbid felspar. The rock may be described as a biotite-augite- 

 oligoclase-porphyrite. 



Certain portions of this mass of rock contain brick-red crystals of 



