70 PKOP. J. W. JFDD ON^ TEETIAEY 



felspar, and sometimes of grains of olivine, we seldom, if ever, find 

 porphyritic crystals of augite or of the hornblende-variety known as 

 " basaltine." In this respect the rocks of this area differ in the 

 most striking manner from the basic rocks of other petrographical 

 provinces, such as that of Bohemia, for example. Another feature 

 by which the basic rocks of the Brito-Icelandic province are distin- 

 guished is the absence of nodules composed of olivine and enstatite ; 

 such nodules are of the most common occurrence in Bohemia, the 

 Eifel, and the Auvergne, but, so far as my own experience goes, 

 they are wholly wanting in the district we are considering. The 

 frequency with which the different varieties of the ophitic structure 

 are exhibited is also another distinctive character of the basic rocks 

 of this petrographical province. 



Porphyritic Gabhros. — In Skye and Ardnamurchan, we find ex- 

 amples of gabbros in which crystals of felspar, sometimes from one 

 to two inches in length, appear to have crystallized before the mass 

 of crystals composing the groundwork of the rock. In some cases 

 the earlier separation of these felspar-crystals is proved by the fact 

 that they have cracked along certain planes of weakness, allowing 

 the pyroxenic material to penetrate into them and crystallize there 

 (see Plate YII. fig. 1). Such porphyritic gabbros appear to be 

 exact analogues in the basic series of the porphyritic granites (like 

 that of Shap, for example) in the acid series. 



Porphyritic Dolerites. — The varieties of these are very numerous, 

 and among them may be detected the exact analogues of rocks 

 which have been designated " labradorit-porphyr," and " diabase- 

 porphyr " in Germany, and also of those which have received the 

 name of " labradorites " from the French geologists. 



The porphyritic varieties of the granulitic dolerites contain, 

 scattered through their midst, broad, platy, felspar individuals, like 

 those characteristic of the gabbros ; and these large felspar- crystals 

 not unfrequently exhibit the zonal structure. In some cases the 

 outlines of these porphyritic felspar-crystals are clear and well 

 defined, but not unfrequently they are irregular in form, and would 

 seem to have suffered mechanical injury. Sometimes the crystals 

 are seen to be split along planes of weakness, and masses of the 

 material which has crystallized as augite have been forced into the 

 cracks (see Plate YII. fig. 2). 



Examples also occur of porphyritic varieties of the ophitic 

 dolerites ; but these, I think, are less frequent than the dolerites 

 with porphyro-granulitic structures. There is one very remarkable 

 variety of the porphyritic structure exhibited by the ophitic dolerites 

 of Pair Head, Co. Antrim, which I shall describe more particularly 

 in the sequel. 



Beautiful examples of the porphyro-granulitic structure are 

 exhibited by the intrusive dolerites in many parts of the area under 

 consideration. I may especially instance those of Dun-da-gu in the 

 Isle of Mull, of Ardnamurchan, and of the Carlingford Mountain in 

 Ireland. Similar structures are exhibited by some of the massive 

 lavas, such as that of Eu Geur in the Isle of Skye. 



