114 Mr. Brycb ok the Geological Structure of Roseneath. 



3. The geological structure of the peninsula is very simple. The 

 upper and middle portions are composed of micaceous and clay slates, 

 while old red sandstone occupies the southern part. These rocks are portions 

 of the great bands of sedimentary strata which traverse Scotland from sea to 

 sea, in a direction parallel to the principal axis of the Grampians, and to 

 the great Caledonian valley. They deviate very little from their ordinary 

 type, and it is therefore unnecessary to enter into any lengthened de- 

 scriptions. The usual varieties, depending upon the varying proportions 

 of the constituent minerals, occur abundantly, but in no definite order, 

 and without, much continuity. Thus the clay-slate series exhibits beds of 

 flinty slate often approaching to quartz rock, of highly bituminous 

 slate, and of coarse grained compact thick bedded slate, mixed up 

 irregularly with the commoner kinds, such as coarse and fine roofing 

 slates and a semi-crystalline silky slate, passing into chlorite or talcose 

 schist. All these varieties ai-e well seen on the road-side between Rose- 

 neath and Kilcreggan, and on the shore between the latter place and 

 North AUey. On the same coast thick cotemporaneous beds and also 

 veins of quartz occur, in the cavities of which rock-crystal is often met 

 with. Roofing slate of good quality is obtained from several quames ; 

 but neither in this rock nor in the mica slate which underlies it, have 

 any indications of metallic ores been noticed. Iron pyrites occurs in 

 the slate rocks in many places. Beds of quartz containing rock-crystal 

 occur frequently in the mica slate, and are well seen on the road-side to 

 the west of the village of Gareloch-head. To the east of Tom-na-hary 

 hill, I found crystals of schorl in a variety of mica slate, containing very 

 little quartz. 



4. In the accompanying map, these two slate rocks are marked as 

 separated by a definite boundary; but in nature no such distinction exists; 

 the transition is in fact so gradual, that it is impossible to say where the 

 micaceous series terminates, and the argillaceous commences. Towards 

 its outer boundary the mica slate begins to assume the character of 

 chlorite schist, and to contain occasional beds of fine roofing slate, the 

 true mica slate still constituting the greater part of the mass. Farther 

 out, the argillaceous and chlorite slates begin to prevail, so that the 

 micaceous beds may be said to be subordinate to them; and thus through 

 oft repeated alternations, we at last reach the true clay slate series. 

 We can conceive, therefore, of a certain middle line, along which the 

 strata partake equally of both characters ; it is this imaginary line 

 which in such a case may fairly represent the boundary. These 

 remarks are equally applicable to the other slate rocks of the district, 

 and indeed to all the rocks of this class in the West of Scotland. 

 In order to explain the mode in which this gradual loss of a marked 

 character, and assumption of one considerably different was brought about, 

 it is necessary to remember that the slate beds were originally deposited 

 from the sea, layer over layer, in the state of silt or fine mud, and after- 

 wards exposed to great heat, combined with pressure. A slight change in 



