192 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



blue colour ; the slates often bluish or greenish grey, weathering 

 brown on exposure. Both rocks contain iron-pyrites in cubical 

 crystals or disseminated in small grains, and also scales of mica. 

 The slates show very distinct division-planes, or foliation, with a 

 very predominant dip at 70° to 1ST. 10° W., but the inclination 

 ranging from 55° near Comrie to 85° on the north near the syenite, 

 and the direction of the dip from N. 15° E. to N. 55° W. 



Before noticing some other peculiarities in the slates, it is better 

 to complete the section by describing the granite, or rather syenite, 

 in the north. This rock rises high up on the declivities of Ben 

 Chonzie (2922 feet high), and occupies a great part of the valley of 

 the Lednock. Generally it is a fine-grained syenite, composed of abun- 

 dant felspar and hornblende, with scales of black mica and a little 

 quartz. In some varieties, as that used in building the Melville 

 Monument, the hornblende is in great measure replaced by mica, 

 and the stone has then a white or grey colour. It has been com- 

 pared to the grey granite of Aberdeen, but the resemblance is 

 merely superficial. In the Comrie syenite the felspar is more pre- 

 dominant, and often forms distinct porphyritic crystals ; hornblende 

 seems always more or less present, and the quartz is far more subor- 

 dinate than in the grey Aberdeenshire granite. In truth, it seems 

 more closely related to the felspar-porphyry or trap group, than to 

 the true granites of the central Grampians. Its peculiar character 

 is better seen in other varieties, which are fine-grained mixtures of 

 red felspar and green hornblende, with very little quartz or mica. A 

 vein or mass of this kind occurs in Dunmore Hill, near the Monu- 

 ment, chiefly composed of red crystalline felspar*. 



Some of the effects of the syenite on the slate near it are very 

 interesting. It appears occasionally to send veins of considerable 

 extent far into the midst of the slate mountains, and finer veins are 

 seen ramifying through the slate irregularly in various directions. 

 On the position of the slate it does not appear to have produced 

 much change, as the beds near it are on the whole conformable in 

 dip and strike to those more remote from it. More marked is the 

 change in the mineral character of the rocks as seen in tracing them 

 along the Lednock from Comrie to the vicinity of the syenite. As 

 they approach the igneous rock the slates gradually become harder 

 and more crystalline. Then lenticular masses of red felspar and 

 quartz, like a binary gneiss, become intermixed with layers, more or 

 less continuous, of the blue slate, till at length the whole rock is 

 converted into a fine-grained gneiss. Even the coarser greywackes 

 are so altered in aspect as, in many places, to be readily mistaken for 

 the syenite. 



11. Bedding or Cleavage (?). — In examining the section just de- 

 scribed, and other sections in the slates, considerable difficulty is 

 often felt as to the true character of the division-planes seen in them. 



* See, for the position of this syenite, Dr. Macculloch's Geological Map of Scot- 

 land, or my own Map. It is omitted, along with the other igneous rocks in this 

 vicinity, in the " Sketch of a New Geol. Map," &c, by Sir R. I. Murchison and 

 Mr. Geikie. 



