508 Alfred Harker — Igneous Rocks of Ski/e. 



been a certain critical epoch, at which in some places basic and acid 

 rocks were intruded almost simultaneously, the basic, however, being 

 slightly the earlier. Remarkable reactions resulted between the two 

 rocks so intimately associated. Here belong : — 



Composite sills and dykes, composed of basic and acid rocks, 

 usually with triple symmetry ; occurring along a belt outside the 

 border of the Eed Hills. 



(3) Phase of Minor Intrusions in the form of sills, sheets, and 

 dykes. Eesumption of regional activity in a new form (intrusive 

 instead of extrusive) ; local activity at certain epochs. Waning 

 intensity indicated during this phase by generally diminishing volume 

 of intrusions, both individually and as groups, and, at least in the 

 Local Series, by intervals of quiescence. 



Begional Series. — Eocks still exclusively basic and (exceptionally) 

 sub-basic, so that no law of chemical variation in time can be laid down. 



(a) Great group of basic sills. These are by far the most important 

 intrusive rocks in the whole suite, making up more than half of the 

 total thickness of the basaltic group over most of the area, besides 

 appearing in considerable force in the underlying Jurassic. Their 

 intrusion constituted the first episode of the Phase of Minor Intrusions. 

 They are here included in the Eegional Series as having clearly no 

 relation to the special focus of Central Skye. They are most 

 developed in the north and west of the island, and die out towards 

 the mountains. 



(b) Basic dykes, mostly with directions near N.W.-S.E,, intruded 

 in vast numbers throughout the region at various epochs, the division 

 into successive groups being possible only in a very partial degree. 

 These basic dykes are to be regarded as self-constituted intrusions ; 

 others of earlier dates being merely the feeders of lava-flows and sills. 



Local Series. — Three chief groups, having restricted areas of 

 distribution, each standing in relation with the corresponding 

 plutonic centre. Order of increasing basicity. 



(a) Minor acid intrusions (dykes, irregular sills, etc.). Area of 

 distribution a roughly elliptic tract, centring in the granite of the 

 Eed Hills but extending beyond, with long axis in the general 

 direction of the dykes (N.N.W.-S.S.E.). 



[h) Minor basic intrusions. Area of distribution nearly coincident 

 with the gabbro of the Cuillins. The most remarkable set of 

 intrusions takes the form of numerous parallel sheets inclined 

 inwards, towards the centre of the area. In addition there is 

 a radiate set of dytes, partly feeders of the sheets, partly older; 

 also, much less perfectly developed, a tangential set of dykes. 



(c) Minor ultrabasic intrusions, in the form of a radiate set 

 of dykes ; distributed with reference to the Cuillins, or rather to 

 the south-western half of the Cuillin area, where the plutonic 

 peridotites occur. 



Subsidiary Groups. — There remain certain groups of dykes, of small 

 importance as regards number and magnitude, concerning which more 

 data are needed. They belong in all cases to very late episodes, but 

 their precise places in the sequence have not been satisfactorily fixed. 



