NIC0L SOUTHERN GRAMPIANS, 203 



In the Quartzite N. 20° W. 



In the Mica-slate .. N. 30° E, 



Difference 50° 



The position of the mica-slate has probably been determined by the 

 eruption of the granite of BenMalngage, one of the northern ontliers 

 of the Ben Cruachan Monntains. Bound this granite a zone of gneiss 

 is laid down; but I had not an opportunity of examining this wild 

 region so as to speak confidently of its position. 



The most remarkable feature in the rocks of this region is the 

 double foliation or structure which they exhibit. Nowhere have I 

 seen this peculiarity more strongly marked in the crystalline strata. 

 In some specimens, layers of a more quartzy nature alternate with 

 others that are more micaceous. In the quartzose layers only one 

 set of lines is seen, which may be regarded as the true bedding. In 

 the more micaceous layers, which on the whole are parallel to the 

 former, there is a second set of lines oblique to the first, and pro- 

 duced by the alternation of folia of mica and quartz. Both sets of 

 lines are of the same nature, produced by the alternation of different 

 mineral substances, and the chief distinction is that the first is more 

 continuous than the second, which is cut off by the quartz -layers 

 even where very thin. In this respect they resemble the bedding 

 and false bedding in ordinary strata. These lines differ essentially 

 from cleavage, either in a rock or simple mineral, inasmuch as 

 their number and position are definite, whereas in cleavage (mark- 

 ing only a minimum of coherence in a certain direction) the mass 

 separates along parallel planes indefinite (or rather infinite) in num- 

 ber and position. This peculiar double structure is not confined 

 to the mica-slate of this locality, but is well seen also in the 

 similar beds near Loch Leven on the north, and Loch Craignish on 

 the south. It must form an element of much importance in all 

 speculations on the origin and history of the rock. The two struc- 

 tures so closely correspond, that we can scarcely doubt that the cause 

 producing the foliation of the rock on the great scale has also pro- 

 duced this cross foliation on the small scale. If the one be an effect 

 of crystallization,- or of mechanical action, subsequent to the deposi- 

 tion of the beds, so must also be the other. It is also of conse- 

 quence as proving that the foliation of the mica -slate may not in 

 all cases coincide with the bedding, but be more or less oblique to it. 

 So much is this the case, that occasionally the inner foliation is highly 

 convoluted or twisted, whilst the outer planes of stratification remain 

 more nearly even or plain. From not observing this fact, many in- 

 accurate determinations of dip and direction have been given. 



20. Loch Leven and Glencoe. — North of Loch Creran lie the valleys 

 of Loch Leven and Glencoe, so well known for their wild and magni- 

 ficent scenery, and so interesting to the geologist from the variety of 

 their igneous rocks. To a superficial observer, merely walking along 

 the roads, the sections may seem simple enough ; but on following 

 them out into the upper parts of the mountains, where the rocks 

 are more fully exposed, many anomalies appear. Therefore, though 



