CHARACTERS OF A SEEIES OF SCOTCH ROCKS. 161 



found myself in this difficulty with a few of the specimens described 

 in this paper. They may be rather exceptional instances of the one 

 or the other type, and with that conclusion I must leave Dr. Hicks 

 to classify them. They will neither contradict nor confirm strongly 

 any stratigraphical theory which he may have formed. The above 

 difficulty may be partly due to the fact that in certain cases the High- 

 land rocks have undergone great local crushing, I believe, always 

 in the vicinity of faults. They thus present a fragmental structure ; 

 and it is by no means easy when some mineral changes have sub- 

 sequently taken place to decide whether we are dealing with a rock 

 of clastic origin, in the ordinary sense of the term, i. e. resulting from 

 the denudation of one of the older gneisses, or with one of the 

 latter, which has been crushed in situ and recemented. ~No doubt 

 the point could in every case be settled by making a larger collection 

 of specimens, or by having several slides cut from different parts of 

 the same specimen ; but I cannot venture always to decide it from 

 the examination of a piece no bigger than a shilling, and it hardly 

 seems worth while, where a crucial point is not involved, to go to 

 considerable expense. Nowhere have I seen such numerous and 

 marked instances of this local crushing as in the Highlands ; but I 

 have observed nothing to countenance Mr. ICallet's theory of vul- 

 canicity. 



It is by no means impossible that the apparently close resemblance 

 occasionally observed between the above-named two groups of rocks 

 may be due to the fact that the older has in many cases supplied 

 the materials of the newer. Under these circumstances, if the 

 latter had been exposed to much pressure and some chemical change, 

 it would be by no means easy to separate the one from the other, 

 even under the microscope. jSTow the Torridon sandstone and quartz- 

 ite in the Loch-AEaree region certainly derive the bulk of their 

 materials from the old gneiss rocks described below as the third 

 type ; but the flaggy " newer gneisses," such as those in the escarp- 

 ment near Loch Maree, seem to be made up of the debris from a 

 schistose series, like that of Ben Pyn. For instance, the quartz in 

 the older gneiss is very full of minute enclosures, many of them 

 resembling irregular empty cavities, though occasionally very small 

 bubbles may be detected, especially in the more minute and regular 

 in form; so is that in the Torridon and quartzite (as a rule). The 

 quartz in rocks of the Ben-Pyn type has comparatively few of these 

 enclosures, so has that in the newest series. Mica also is common 

 in the latter two, rarer in the former two rocks. 



The third type, while agreeing with those described under the second 

 head, as being metamorphosed to the highest degree, appears to 

 differ in respects which can hardly be due to a mere prolongation of 

 the metamorphic action. The bedding of these rocks is ill marked ; 

 they are coarsely crystalline and often granitoid in aspect, being then 

 difficult to distinguish from rocks of igneous origin ; and the same is 

 true of their microscopic structures. In such cases, in the present 

 stage of our knowledge (though I do not think it will be so always), 

 we must be content to be sometimes uncertain whether we have 



