1849.] GUMMING ON THE TERTIARIES OF THE MORAY FIRTH. 13 



as well as of the boulders in it, is a well-known circumstance, and is 

 in fact characteristic of the formation. A mass of gravel boulders 

 and sand not having these marks, may almost as a certainty be set 

 down in the category of modern raised beaches, or may be looked 

 upon as a re-formation of the true boulder series in those beds, which 

 in pre\dous memoirs I have termed the drift-gravel. The drift-gravel 

 I look upon as having originated in a rising condition of the land ; 

 the boulder-clay during a period of subsidence. Now it appears to me, 

 that this very circumstance of the rocks i7i as well as under the 

 boulder-clay being thus grooved and polished, is in itself a strong 

 evidence of subsidence. The grooving must have taken place prior 

 to the covering up of the fundamental rock, and the same must be 

 true of each successive scratched fragment of rock in the superior 

 mass*. 



The extremely local character also of the great mass of the boulder- 

 clay series seems to direct to a similar view, and to indicate in many 

 instances the direction of the general drifting current. In a paper 

 read before this Society in February 1846f, I pointed to the fact 

 that the colour of different portions of it, even within a very limited 

 area in the Isle of Man, is different, and attributable to the different 

 colours of the rocks over which the drifting current had passed. 

 Through the kindness of George Kemp, Esq., M.D. of St. Peter's 

 College, Cambridge, I have since more fully had established, by 

 chemical analysis of these boulder-clays of the Isle of Man, the fact 

 of their very local origin | ; and the same fact is equally evident 

 along the shores of the Moray Firth, and determines that the drifting 

 current had a direction generally from west to east ; I say a general 

 direction, because at any particular spot the direction would be 

 necessarily dependent upon the shape and direction of the sounds 

 and straits through which the drifting currents were forced. 



For instance, whilst in the immediate neighbourhood of Inverness 

 the current appears to have set along the great Caledonian Valley from 

 S.W. to N.E., we find in the Cromarty Firth the scratchings to 

 be due E. and W., and further north at Braambury Hill, and Brora, as 

 stated by Sir R. I. Murchison, they are from N.W. to S.E. 



As respects colour and mineral contents, the same thing is evidenced 

 of the general direction of the drifting current. In the neighbour- 

 hood of Gamrie, we have fragments of the lias beds of the Moray 

 Firth mixed with the ichthyolite beds of the immediate district : as 

 we advance westward along the coast, we get fragments of the corn- 

 stones and upper beds of the old red sandstone ; on the Black Isle 



* Mr. Miller with his usual acumen pointed out to me the extreme value of the 

 fact which I had often observed, that the majority of the fragments of locks in the 

 boalder clay are scratched and furrowed in the direction of their length. They 

 were evidently not rolled along loosely in water, in which case they would offer 

 the surface of greatest resistance to the wave, and be scratched diagonally, but 

 pushed along as by an ice-raft grinding heavily over them, and therefore sliding 

 on in the direction of their longer axes. 



t Quarterly Journal of the Geological Society, vol. ii. part 1. p. 339. 



X See my ' Isle of Man, its History,' &c., p. 305. 



