OF THE EARTH'S SURFACE. 183 



many yards in length. Occasionally, single scratches, and pa- 

 rallel sets of them, deviate by five or six degrees from the ge- 

 neral direction ; but the important circumstance is, that such 

 deviation is rare, the very great majority of both sets agreeing 

 in parallelism with each other, and with the general direction, 

 not only of the scoops and grooves of the rock upon which 

 they occur, but also of the ridges and large features of the di- 

 strict. A rock covered with these furrows, has externally an 

 appearance greatly resembling what is called Slickenside, with 

 this difference, that in the slickenside, we can always discover 

 some proof that one portion of the main rock has performed a 

 small slide upon the other ; whereas, in this case, every thing 

 shews, that the rock under consideration has stood firm, and 

 has been abraded by a number of bodies in motion. The cir- 

 cumstance just mentioned, of occasional deviations from paral- 

 lelism, seems also to distinguish this form from the slicken- 

 side ; in which last, I believe, the lines are invariably paral- 

 lel * 



The direction of the stream in the neighbourhood of Edin- 

 burgh, as indicated by the medium result of a number of ob- 

 servations, appears to have been from 10° S. of W. to 10° N. 

 of E., by true bearings taken with a needle, and allowing 

 27^ degrees west of north as the variation ; and I have met 



with 



* They are also distinguished by this, that in the case of Slickenside, the rock 

 has always upon it a crust of calcareous spar or zeolite, which seems to have oc- 

 cupied a vein. Both sets seem to have been the result of mechanical action. The 

 slickenside has been produced, I conceive, in the Plutonic regions, at a time when 

 the mass in general has been so far cooled, as to be solid, though sufficient heat 

 has remained to keep the fusible matter, as carbonate of lime or zeolite, which 

 filled the vein, in a state at least of semifusion ; so that in the sliding of one mass 

 on the other, it has acted like grease in the wheel of a carriage : this theory ac- 

 counts for the parallelism of the lines of slickenside. 



