September 7, 191 1] 



NATURE 



vading rocks so different from itself as gabbro 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 facies of our 

 Tertiary plutonic rocks are perhaps significant in that they 

 occur near the northern and southern limits of the prin- 

 cipal 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 alkaline 

 mineral known at only one spot in Skye. The micro- 

 perthitic granites of Arran do not carry riebeckite, but it 

 is found in the well-known rock of Ailsa Craig, farther 

 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 felsite 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 analcime, 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 pro- 

 ducts, formed at the expense of the felspar by the action 

 of volcanic water, while the rocks were still at a some- 

 what high temperature. Subsequent reconsideration has 

 led me to regard these minerals rather as primary con- 

 stituents 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 

 observations. Analcime in particular is not always con- 

 fined to the steam-cavities, but in some cases occurs inter- 

 stitially 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 pleo- 

 chroism. From these and other features it appears that 

 this group of rocks reveals on examination decided, though 

 not very strongly marked, alkaline affinities. 



Volcanic rocks of other than basaltic composition are 

 not largely developed. They include both rhyolites and 

 trachytes, the former without very distinctive characters, 

 but the latter falling naturally into the alkaline division. 

 In describing 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 ex- 

 posed 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 widespread group of basic sills. The 

 common non-porphyritic dolerite sills have, in most dis- 

 tricts, 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 represented 

 among the alkaline rocks of the Scottish Carboniferous 

 province. Mugearite, a type still richer in alkalies, is like- 

 wise common to the two provinces. As we approach the 

 limits of the principal belt of activity, alkaline character- 

 istics 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 farther north, in the Shiant Isles, 

 analcime enters, and even, according to a record of 



NO. 2184, VOL. 87] 



Heddle, nepheline. 1 At the other extreme, in southern 

 Arran, occur the analcime-dolerite sills of Clauchland and 

 Dippin. 



The regional basic dykes, which are mostly posterior in 

 age to the sills, exhibit more variety of composition. 

 Some with abundant porphyritic felspars resemble the 

 Markle type of dolerite, and there are others of mugearitic 

 nature, but these are only a minority. In Argyllshire 

 there are basic dykes with purple pleochroic augite, and 

 even some of camptonite and monchiquite ; but these latter 

 at least I should exclude as being probably of late 

 Palaeozoic age. s The undoubtedly Tertiary dykes, how- 

 ever, exhibit a variety which can be explained only as the 

 result of repeated differentiation. The distribution of some 

 of the groups indicates the existence at this late stage of 

 subsidiary centres of differentiation, distinct from the 

 plutonic centres. Thus trachyte dykes are found especially 

 throughout a tract extending from the south-western part 

 of Skye through the middle of Argyllshire, while there is 

 an isolated area of these dykes about Drynoch, on the 

 opposite side of the Skye mountains. Here we have an 

 evidently alkaline type. On the other hand, there are 

 rocks which, taken by themselves, must be assigned to the 

 calcic division. Augite-andesites, for example, are well 

 known, especially in parts of western Argyllshire, in Arran 

 and the Cumbraes, and in the outlying districts of the 

 north of Ireland, Anglesey, and the north-east of England. 

 That these rocks have arisen as products of a subsidiary 

 differentiation we have in some cases almost ocular demon- 

 stration ; for in Arran and elsewhere augite-andesites are 

 found in remarkably intimate association with comple- 

 mentary types, often pitchstones of alkaline composition. 



Even from so brief and imperfect a sketch we may, I 

 think, draw some conclusions which have a wider applica- 

 tion. This province exemplifies at once the two main 

 tectonic types, and also comprises representatives of the 

 two great branches of igneous rocks. Those rocks which 

 are related to broad movements of Atlantic type indicate 

 a parent magma of decided, though not strongly marked, 

 alkaline nature ; while those related to local movements of 

 Pacific type clearly come from a calcic magma. There are 

 some facts which suggest that the rocks tend to become 

 more alkaline as we recede from the chief centres of 

 activity, and this suggestion applies to some calcic as well 

 as alkaline groups of rocks. Finally, it appears that the 

 relative simplicity of arrangement was disturbed at a late 

 stage by the effects of subsidiary differentiation, the 

 province tending then to break up into districts related to 

 new centres. Operating upon an initial magma not very 

 strongly characterised, this later differentiation has even 

 given rise to aberrant rock-types which overstep the petro- 

 graphical boundary-line between the two branches. 



Petrogenesis and Systematic Petrography. 



From such considerations as I have hastily passed in 

 review, it is evident that a survey of igneous rocks as they 

 actually occur in the field leads to a conception of their 

 mutual relationships very different from that embodied in 

 the current schemes of systematic petrography. It may be 

 of some interest, in conclusion, to expand this remark a 

 little farther, although I am sensible that in so doing I 

 lay myself open to the charge of vain speculation. 



From the petrogenetic point of view, the most funda- 

 mental division among igneous rocks is that between the 

 alkaline and calcic branches. This result, independently 

 arrived at on petrographical grounds by several authori- 

 ties, seems to be firmly established by the broad distribu- 

 tion of the two branches in different regions of the globe. 

 But, if this argument be admitted, it follows that the 

 next step in a natural grouping of igneous rocks should be 

 suggested by a comparison of the characteristics of the 

 various provinces into which the great regions divide. 

 Many of these provinces have now been partly studied, and 

 their special characteristics can often be expressed in 

 concise terms : e.g. among alkaline rocks the relative pro- 

 portion of potash to soda may be a characteristic common 

 to a whole province. More precisely, by averaging the 



1 The dolerite here is intimately associated with ultrabasic rocks, as has 

 be"n described by Judd. 



2 A like remark applies to the highly alkaline dykes of the Orkneys, which 

 do not agree even in direction with the Tertiary suite. 



