364 



SCIENCE 



[N. S. Vol. XXXIV. No. 873 



salts of Skye and the Small Isles some 

 years ago, I regarded the zeolites as sol- 

 fataric 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 reconsid- 

 eration has led me to consider these min- 

 erals rather as primary constituents of the 

 rock, crystallized directly from the final 

 residual magma, which had become rela- 

 tively 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 ex- 

 amples from Mull and Skye has enabled 

 me to confirm and extend his interesting 

 observations. 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 ap- 

 pearance 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, alkaline 

 afBnities. 



Volcanic rocks of other than basaltic 

 composition are not largely developed. 

 They include both rhyolites and trachytes, 

 the former without very distinctive char- 

 acters, but the latter falling naturally into 

 the alkaline division. In describing for- 

 merly a group of rhyolites and trachytes on 

 the northern border of the Cuillins, I con- 

 nected it with the neighboring plutonie 

 center, but I have since found other 

 trachytes in Skye: there is a fine develop- 

 ment exposed in the glen above Bracadale. 

 From this, and from the situation of the 

 Antrim rhyolites, I infer that these fel- 

 spathic and acid lavas, though distributed 



sporadically, belong to the regional or At- 

 lantic 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 por- 

 phyritic 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 likewise com- 

 mon to the two provinces. 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 

 farther north, in the Shiant Isles, analcime 

 enters and even, according to a record of 

 Heddle, nepheline.^ At the other extreme, 

 in southern Arran, occur the analcime- 

 dolerite sills of Clatichland and Dippin. 



The regional basic dykes, which are 

 mostly posterior in age to the sills, exhibit 

 more variety of composition. Some with 

 abundant porphyritie felspars resemble 

 the Markle type of dolerite, and there are 

 othere 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 monchi- 

 quite ; but these latter at least I should ex- 

 clude as being probably of late Paleozoic 

 age.'' The undoubtedly Tertiary dykes, 

 however, exhibit a variety which can be 



- The dolerite here is intimately associated with 

 ultra-basic rocks, as has been described by Judd. 



^ A like remark applies to the highly alkaline 

 dykes of the Orkneys, which do not agree even in 

 direction with the Tertiary suite. 



