410 PROP. J. W. JITDD ON THE TEETIAET AND 



enstatite into bastite, and olivine into serpentine. These changes, 

 which are quite distinct in their nature and their origin from 

 Schillerization, may he greatly modified, however, by the alteration, 

 due to deep-seated action, which the minerals have previously un- 

 dergone. 



As diallage is only an altered form of augite, it is impossible to 

 maintain as a separate class rocks whose only distinction is the 

 presence of that variety. Hence, as many petrographers have 

 admitted, it becomes difficult to accept the mineralogical distinc- 

 tion between gabbro and diabase. The name gabbro is so con- 

 venient, that its retention is advocated for all the most perfectly 

 holocrystalline (granitic) varieties of basic rocks, whether the augite 

 is in its normal condition or its Schillerized form (diallage). The 

 name diabase may be more conveniently employed for altered forms 

 of dolerite. 



In many cases the process of Schillerization has resulted in the 

 conversion of the olivine into a black and opaque substance, often 

 mistaken for magnetite. Many of the gabbros supposed to contain 

 no olivine in reality have their olivine in this curiously altered 

 state. The class of the olivine-gabbros is much larger than is 

 usually supposed, and the group of gabbros without olivine is 

 proportionally restricted. 



It has been shown that in the Western Isles of Scotland there 

 exists a series of ultra-basic rocks of Tertiary age which exhibit all 

 the essential features of the ultra-basic rocks of pre-Tertiary age, and 

 like them may be classed under the varieties of dunite, Iherzolite, 

 picrite, eucrite, and troctolite. These rocks are most intimately 

 associated with the gabbros and dolerites of the district, an 

 association which finds an exact parallel in the case of their older 

 representatives. 



The study of the remarkable changes which the minerals of 

 these rocks undergo, as they are traced to successive depths from the 

 original surface, is greatly facilitated by the fact that, in consequence 

 probably of the late period of their eruption, they have suffered but 

 little from agents acting from the surface. In many cases the 

 felspars exhibit no trace of kaolinization, the augites are fresh and 

 show no signs of uralitization, and the olivines are not in the least 

 serpentinized ; thus the changes which are due to the action of deep- 

 seated waters are not in the least degree complicated with, or 

 concealed by, alteration of a totally different character and 

 origin. 



But in the masses of peridotite of Palaeozoic age which are 

 scattered about Scotland quite opposite conditions prevail. The 

 dunites are converted into serpentine-rock, the Iherzolites into 

 bastite-serpentine, the enstatite-rocks into bastite-rocks, and the 

 augite-picrites into hornblende-picrites. But, in all these cases, a 

 careful study of the altered materials shows that originally they 

 were identical in mineralogical constitution and in structure with 

 the peridotites of Tertiary age. 



In the scyelite of Caithness we have a very interesting example 



