Ch. VI.] PRIMARY ROCKS. 109 



structure of these rocks. That this circumstance is not so 

 apparent in other countries, may be attributed to the presence 

 of secondary and tertiary rocks, as well as of recent debris, 

 by which the surface has assumed a new and different feature. 



For farther particulars on this subject the Cornish Trans- 

 actions may be consulted ; but we must not omit to observe, 

 that the laminae of the schist very frequently correspond with 

 the ascending and descending curves, which are parallel 

 with the granite : or, in other words, in traversing the slate 

 at right angles with the granite, the laminae dip in the same 

 direction as the slopes of the hills : and that the strata forming 

 the limb of the curve nearest to the granite are commonly 

 thinner than those on the other side of the curve, and are 

 more abrupt, dipping at a greater angle ; in consequence of 

 which structure, it often happens that the thin and highly 

 inclined strata, at the summit of the hills, have been entirely 

 degraded; whilst the more extended strata are persistent, 

 exhibiting an underlie conformable with the granite. In like 

 manner, the small islands on the coast, which frequently con- 

 sist of the upper parts of such individual curves, have some- 

 times lost the highly inclined layers by the action of the waves, 

 only retaining those strata which dip from the shore, and 

 which correspond, or not, with the inclination of the nearest 

 mainland, according as this is composed of the ascending or 

 descending portion of the curves. The north coast of Corn- 

 wall, particularly at Portreath, and the adjacent coves, well 

 illustrates this disposition of the strata. 



It is time, however, to turn our attention to the structure 

 of the primary schistose rocks of other countries. 



The disposition of the beds of gneiss in the Western Isles 

 of Scotland is very irregular, more particularly those of the 

 granitic species of this rock. Towards the southern and 

 western parts of Coll, the gneiss is often arranged in regular 

 beds, of which the alignement can be traced for a consider- 

 able space, straight and free from all that disturbance which 

 marks the beds of Tirey. Towards the middle and northern 



