41 



silky slate, passing into chlorite or talcose schist. All these varieties 

 are well seen on the road-side between K-oseneath and Kilcreggan, 

 and on the shore between the latter place and North Ailey. 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 quarries ; 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 crystals of schorl are found in a variety of mica 

 slate, containing very little quartz. 



30. On a geological map these two slate rocks would be 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 com- 

 mences. 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 con- 

 siderably different was brought about, it is necessary to remember 

 that the slate beds were originally deposited from the sea, layer over 

 layer, in a state of silt or fine mud, and afterwards exposed to great 

 heat, combined with pressure. A slight change in the nature of the 

 sediment, or in the amount of heat or pressure, would be sufficient 

 to produce the want of uniformity, and the variations from a definite 

 type, which we now observe. The arrangement usually is, upper 

 green or chlorite slate, middle dark coloured clay slate, and inferior 

 mica slate, in some tracts passing into gneiss, and extending inward 

 to the Grampian granite. The mica slate has a remarkable structural 

 arrangement which distinguishes it from the two others ; its crys- 

 talline laminae are simple foliations, and form only one set of divisional 



