378 TRANSACTIONS OF SECTION C. 



interesting consequence. In many places a granite magma, invading rocks so 

 different from itself as gabbi'o or eucrite, has caused energetic mutual reactions, 

 and a set of hybrid rocks has been produced, which serves in a limited sense to 

 fill the gap in the series. 



The only known exceptions to the calcic fades of our Tertiary plutonic rocks 

 are perhaps significant in that they occur near the northern and southern limits 

 of the principal belt of activity. The massive gently-inclined sheets of granite 

 and granophyre which make up part of the southern end of Raasay consist 

 largely of microperthite, and contain abundant riebeckite, a distinctively alka- 

 line mineral known at only one spot in Skye. The microperthitic granites of 

 Arran do not carry riebeckite, but it is found in the well-known rock of Ailsa 

 Craig, further south. 



The local groups of minor intrusions — acid, basic, and ultrabasic — related to 

 the several plutonic centres have the same calcic facies as the plutonic rocks of 

 which they are satellites. It appears, however, that they sometimes tend to a 

 more alkaline composition towards the borders of their respective districts. 

 Thus, the Skye granite is surrounded by a roughly oval area, within which are 

 found numerous dykes and sills of f elsite and granophyre, in general augitic ; 

 but on the fringe of the area these rocks give place to orthophyres, with biotite 

 or hornblende, and to bostonites. 



Turn now to the rocks of regional distribution. The most important are, of 

 course, the basalt lavas. They are all felspar-basalts, but a very general feature 

 is the filling of their numerous amygdaloidal cavities with zeolites, such as anal- 

 cirne. natrolite, chabazite, and stilbite. These minerals are certainly not mere 

 weathering-products. When I examined the basalts of Skye and the Small Isles 

 some years ago, I regarded the zeolites as solfataric products, formed at the 

 expense of the felspar by the action of volcanic water, while the rocks were still 

 at a somewhat high temperature. Subsequent reconsideration has led me to 

 consider these minerals rather as primary constituents of the rock, crystallised 

 directly from the final residual magma, which had become relatively enriched in 

 water by the abstraction of the anhydrous minerals. Such was the conclusion 

 reached by Mr. James Strachan for the Antrim basalts, and a study of examples 

 from Mull and Skye has enabled me to confirm and extend his interesting obser- 

 vations. Analcime in particular is not always confined to the steam-cavities, but 

 in some cases occurs interstitially in the rock, where it is certainly not derived 

 from felspar, and, indeed, has all the appearance of a primary constituent. The 

 augite of these analcime-bearing basalts has in thin slices a purplish tint, with 

 sensible pleochroism. From these and other features it appears that this group 

 of rocks reveals on examination decided, though not very strongly marked, alka- 

 line affinities. 



Volcanic rocks of other than basaltic composition are not largely developed. 

 They include both rhyolites and trachytes, the former without very distinctive 

 chai'acters, but the latter falling naturally into the alkaline division. In describ- 

 ing formerly a group of rhyolites and trachytes on the northern border of the 

 Cuillins, I connected it with the neighbouring plutonic centre, but I have since 

 found other trachytes in Skye : there is a fine development exposed in the glen 

 above Bracadale. From this, and from the situation of the Antrim rhyolites, I 

 infer that these felspathic and acid lavas, though distributed sporadically, belong 

 to the regional or Atlantic suite. 



Consider next the wide-spread group of basic sills. The common non- 

 porphyritic dolerite sills have, in most districts, little that is indicative of 

 alkaline affinities, though chemical analyses show a rather noteworthy amount of 

 soda. In the porphyritic dolerites this characteristic is much more apparent, and 

 indeed these rocks are almost identical with the ' Markle type ' so largely repre- 

 sented among the alkaline rocks of the Scottish Carboniferous province. 

 Mugearite, a type still richer in alkalis, is likewise common to the two pro- 

 vinces. As we approach the limits of the principal belt of activity, alkaline 

 characteristics become well marked even in the common non-porphyritic dolerites. 

 This is shown in Raasay and the northern part of Skye by the coming in of the 

 purple pleochroic augite. while further north, in the Shiant Isles, analcime 

 enters and even, according to a record of Heddle, nepheline. 1 At the other 



1 The dolerite here is intimately associated with ultra-basic rocks, as has been 

 described bv Judd. 



