in the Middle Eegion of Scotland. 171 



holds true of both A aud B. It seems strange that the action 

 of the tide, which might be expected to obliterate the grooves, 

 should be the means of preserving them. Such, however, is 

 the fact; and perhaps it admits of explanation. The agent 

 chiefly concerned in wasting the surface of the granite above 

 the water, appears to be a sort of grey fog (a cryptogram, I 

 suppose), everywhere visible, which takes root upon it, and, 

 falling off, and being renewed periodically, can*ies minute 

 grains of the rock with it, and thus gradually wears down 

 the surface, and obliterates all fine markings upon it. The 

 part beneath the high water line is protected from this vege- 

 tation by the tide, and still more, by a sort of black pigment 

 which the sea deposits upon the rock. To the eye, it has ex- 

 actly the aspect of a coat of paint, and is probably about the 

 100th part of an inch thick. When examined with a lens, it 

 is seen to be divided by innumerable cracks into polygonal 

 facets, from a 40th to a 200tli part of an inch in breadth. 

 This coating is probably permanent, or at least very durable ; 

 and it covers most of the large groovings which are uncjer the 

 high water level, apparently without rendering them less con- 

 spicuous. I found it also on the basaltic clinkstone at Loch 

 Etive (arrow 3) ; but there it seemed to be confined to the 

 parts of the rock near the high water line, and probably to 

 those daily wetted by the spray. It was thick enough to con- 

 ceal the fine striae, which were visible at a greater depth. 



These two granitic masses seem to me to present a crucial 

 test for Sir James Hall's theory, and entirely to disprove his 

 fundamental proposition. From the facts detailed, two con- 

 clusions inevitably result : First, that the agent which pro- 

 duced the grooves moved from east to west ; secondly, that 

 no agent capable of producing groovings, and acting contem- 

 poraneously, moved in the opposite direction, or from west to 

 east. We have here the advantage so seldom obtainable, of 

 proving a negative. Sir James Hall, according to his theory, 

 would have said that the grooves on the east ends of A and 

 B were produced by the recoil wave, thrown back by the 

 mountains ; but if the recoil or secondary wave, forming but 

 a fraction of the great debacle, had sufiicient power to do this, 

 the direct and primary wave which immediately preceded it, 



