various parts of Scotland. 439 



forces have acted, Independently of the neighbouring ones. It is 

 equally evident, that a mere state of softness in the stratum of schist 

 is insufficient to account for this partial and complicated effect, but 

 that it must have taken place when the rock was in a state of 

 tenacious fluidity. The effect produced by mechanical disturbance 

 on fluid slags which consist of differently coloured laminae, will il- 

 lustrate my meaning, or, to leave out of the question any illustration 

 which may appear to involve a cause, similar appearances may be 

 produced by the disturbance of tenacious compounds of clay and 

 water, or of other semifluid mixtures. It is not perhaps essential to 

 the igneous theory, that this fact should be explained by an igneous 

 softening ; but it is necessary that the aqueous theory should admit 

 of a disturbing force capable of producing a mechanical effect of this 

 nature, since nothing short of mechanical force acting on yielding 

 matter is capable of explaining it. 



I may here add a fact somewhat illustrative of these partial con- 

 tortions in schist, which is to be observed occurring in the basalt 

 that forms the hill of Dun Can, in Raasa. Innumerable instances 

 prove to us that the decomposition of basaltic as well as of granitic 

 and other rocks, detects in them that structure which we should in 

 vain seek in the fresh and entire mass. The weathered basalt to which 

 I allude discovers appearances of contortion similar to those now 

 under review, its surface exhibiting parallel prominences, separated 

 by deep lines, which are waved in very complicated curves, and 

 give reason to suspect a mechanical disturbing force, acting in this 

 case also, upon a semifluid mass. As analogous appearances of cur- 

 vature are also of frequent occurrence both in veins of quartz, and 

 in veins of granite which traverse mica slate and gneiss, I have 

 given figures of a few among the most remarkable of the innume- 

 rable specimens to be seen all over Scotland (PI. 31 f). I have 



