AJTNIVERSAET ADDRESS OP THE PEESIDEXT. 93 



tiire, in short, of the rock recalls to mind that of some of the more 

 highly altered quartzites, and is sometimes slightly shadowed out by 

 such fragment al rocks as the Torridon Sandstone, although I do 

 not therefore mean to assert a clastic origin. Occasionally the 

 peculiar association of quartz and felspar, which might be designated 

 microdendritic, is to be seen ; I maj' also remark, without wishing to 

 attach too much value to the fact, that in these gneisses microcline 

 felspar is remarkably abundant. The hornblendic rocks, presumably 

 interstratified with these gneisses, are also coarsely crystalline ; 

 indeed this is the general characteristic of the series, whether its 

 members be quartzose, felspathic, micaceous, calcareous, or other- 

 wise. Even the crystalline limestones have a character of their own, 

 both in their crystalline structure and in the not unfrequent presence 

 of various accessory minerals, such as garnet, malacolite, or other 

 members of the pyroxenic group, mica, &c. 



As the gneisses become more finely grained, the structure de- 

 scribed above is less marked, and the grains of quartz and felspar 

 assume a rather more irregular outline, analogous in some respects 

 to that to be seen among certain of the vein-granites, so that 

 occasionally it is by no means easy, in the hand specimen or under 

 the microscope, to distinguish one of these gneisses, when almost 

 without mica or signs of foliation, from certain of the vein-granites. 

 But, as a rule, in this series, mica is more abundant than in the 

 last, the rocks vary more frequently, and the different members are 

 less coarsely crystalline. This diminution in the felspathic con- 

 stituent seems to become more marked as we proceed upwards ; and 

 where this mineral does appear in rocks presumably of stratified 

 origin*, it has generally an ill-defined outline, an uncharacteristic 

 aspect, and is commonly ill preserved. The aluminous subsilicates 

 now become more frequent, appearing to take its place ; mica is a 

 very common mineral, so that mica-schists are often very abundant. 

 White mica now seems rather commoner than black ; the accessory 

 minerals, such as garnet, andalusite, cyanite, tfec, are often well 

 developed ; but the matrix is, as a rule, not coarsely crystalline. As 

 we proceed onwards, it can no longer be doubted that we meet 

 with signs of original stratification, and with rocks which must be of 

 detrital origin. AV ell-crystallized accessory minerals (especially 

 silicates) seem to become rarer ; the matrix not only is apt to be 

 rather minutely crystalline, but also sometimes seems to contain a 

 residuum of uncrystallized material, carbonaceous and earthy par- 

 ticles occasionally remaining and giving a dirty aspect to the 

 crystalline constituents of the rock. So far as I have observed, and 

 this seems to me very important, these rocks exhibit stratification- 

 foliation, that is, the general direction of the constituent folia is 

 parallel with the apparent stratification of the rock-masses. This, 

 as it affects such large areas, appears to indicate that the dominant 

 pressure at the time of crystallization was vertical rather than 

 lateral. Hence, either the rocks were buried deep beneath other 



* As in the gneisses so comiuon in the Lepontine Alps, the '' Montalban " type 

 of Dr. Sterry Hunt. 



