264 PEOCBEDINGS OP THB GEOLOGICAL SOCIETY. [Feb. 7, 



Dunnet Bay and Sinclair Bay. Another low tract passes up the bed 

 of the Thurso water and along to the east coast by way of Dunbeath. 

 The drift is spread in sheets filling up these troughs and levelling 

 the irregularities of the rocky strata so as to impart a smoother and 

 softer outline to the surface. So far as I saw, it does not form irre- 

 gular mounds and hillocks, neither is it very rough on the surface 

 with erratic blocks ; it seems confined in a great measure to the 

 lower levels, thinning out at altitudes of 100 or 150 feet. The 

 thickness, therefore, varies much, being greatest in those depressions 

 which descend nearly to the sea-level. Thus at Scrabster harbour, 

 in Thurso bay, there are banks of it more than 100 feet high, and 

 in some of the troughs of the south branch of the Dunbeath water 

 it is nearly as thick. Deep masses occupy the hollow from Watten 

 to the Bay of Wick, and also stretch along the Thurso river into the 

 very centre of the county. There is also a good deal of it in the 

 bed of the Forss and at Lybster, but at the Ereswick Burn it is com- 

 paratively thin. In all these places it is of very much the same hue, 

 being of a deep leaden-grey or slate-colour, very dark when moist, 

 and considerably paler when dry, similar, in fact, to the colour of the 

 Caithness flags on which it rests. Occasionally the upper portion is 

 of a browner, more ferruginous tint, which may be owing to the 

 influence of the atmosphere, the percolation of surface-water, or to 

 some other cause. 



Fig. 2. — Scrabster Harhour. 



1. Unstratified pebbly clay with Tery few broken shells. 

 2. Old Red Sandstone. 



The texture, however, varies a good deal in different places. At 

 Scrabster harbour, where it reaches a thickness of more than 100 

 feet, no difference can be perceived from top to bottom ; it is just 

 the same at the base, where it is in contact with the ice-worn sur- 

 face of the subjacent rock, as it is 100 or 150 feet higher up. It 

 shows no stratification nor traces of gradual deposition. It is a coarse 

 gritty mud, exceedingly firm and difficult to pierce, and thickly 

 charged with smaU stones, which are dispersed very uniformly 

 throughout all parts of the deposit. Pebbles of all sizes below that 

 of a man's fist or foot are the prevaiUng dimensions, but stones of 

 from two to three feet in length also occur ; I saw no great erratic 

 blocks. The stones are more or less worn and rubbed, and many of 

 them show the glacial scratches. A few small fragments of shells 

 are dispersed through all parts of the bank, from the bottom to 

 within at least 15 feet of the top, but are by no means common. 

 Such is the character of the section at Scrabster, and it is very much 

 the same along the banks of the Thurso water for several miles up 



