LEWIS. GEOLOGY. 193 



of dark and pale gneiss in pretty equal and parallel 

 laminae, the darker parts abounding in hornblende, the 

 lighter in felspar. Dispersed through this rock are 

 crystalline concretions of felspar, thickly disseminated, 

 and disposed in a laminar direction corresponding with 

 the general structure of the rock. This felspar is splen- 

 dent or nearly glassy, and the concretions, although highly 

 crystalline within, have no external configuration. They 

 are seldom less than a quarter of an inch in diameter, 

 and attain to that of three inches* It is not unusual to 

 meet with crystallized felspar in gneiss, and the appear- 

 ances which it exhibits are often remarkable, as the 

 laminae are generally bent and accommodated to the shape 

 of the crystal. But it rarely in these cases exceeds the 

 fourth of an inch, and seldom possesses that perfectly 

 crystalline aspect which it here displays. This rock is 

 also bent, like most others in the island, and is attended by 

 an accidental circumstance which renders the incurvation 

 particularly striking. This is the falling away of the 

 surrounding parts, which have thus left standing an 

 example, perhaps solitary, of a bent and detached rock. 

 It is about thirty or forty feet high.* 



In the neighbourhood of Stornoway, and in the interior 

 country towards Tongue and Barvas, the gneiss is found 

 passing into common argillaceous schist ; forming a com- 

 pound rock, in its constituents not unlike that which has 

 been described under the head of North Uist, but much 

 less hard in texture. To describe it correctly, it may 

 be said that a felspar much intermixed with quartz forms 

 the one part, and common clay schist the other. Some- 

 times the first substance predominates, at others the 

 last, and as these vary, so the rock passes on the one 

 hand into a gneiss, on the other into common clay slate. 

 It differs from the compound rock of North Uist in 

 being much less hard, the schist indeed generally retaining 



* Plate I. 

 VOL. I. O 



