ROCKS OF THE NORTHERN HIGHLANDS. 363 



apparently taken place. The gneiss is chloritic, its silica having 

 probably taken up a part of the magnesia of the dolomite ; and the 

 dolomite, thus partially decomposed, displays imperfectly the reaction 

 of calcic carbonate. 



On the other hand, the Durness limestone, so far as I have seen, 

 always effervesces freely with acids. 



The chemical distinction between the Durness and Erriboll lime- 

 stones, pointed out by Dr. Anderson and reasserted by Prof. Heddle, 

 thus appears to me to be firmly established. 



In Assynt I observed that the lower half of the Dolomite was of 

 a dark grey colour, sometimes weathering almost black, while the 

 upper part was nearly white. The same difference is equally well 

 marked on Loch Erriboll. This separation into a Lower (c. 1.) and 

 an Upper (c 5 u.) Dolomite is useful as an aid in determining whether 

 or not inversion has taken place. 



The maximum thickness of the dolomite is about 300 feet, or 

 rather less. This estimate does not materially vary anywhere on 

 the line of strike between Ullapool and Erriboll, the local changes 

 being due, I believe, not to thinning but to faulting. 



II. The Relations between these Eormations. 



I describe the districts examined from S. to N"., since the evidence 

 I have to adduce grows progressively stronger in that direction. 



Loch Broom. 



As this is one of the districts in which " a gradually descending 

 series" has been affirmed, I devoted to it in 1881 a very close ex- 

 amination. If there is no break here, we ought to find the same 

 succession wherever we run our sections ; for, within so limited an 

 area, variations of thickness along the strike cannot be considerable ; 

 and it is of course impossible that this cause, though it may account 

 for the absence of a group, should interfere with the true order. 

 Of the sections examined I will describe six. These I take from 

 S. to N. 



1. Section along the shore of the Loch from Ullapool to the S.S.E. 



At about half a mile from the hotel we come to sandstone and 

 conglomerate, which occupy the shore for a considerable distance ; 

 but in the rounded promontory which terminates in Corry Point a 

 gneissic rock suddenly comes in. This is the " serpentine or felspar 

 porphyry " of Nicol : but in the paper of Sir E. I. Murchison and 

 Prof. Greikie* it is described as " serpentinous and felspathic rock 

 containing pebbles of jasper," and it is regarded as " a highly meta- 

 morphosed band of felspathic grit." It is the " Logan Rock " of 

 Dr. Heddle. As it plays a very important part in our inquiry, it 

 is necessary to examine its true nature very carefully. 



I first made the acquaintance of the " Logan Rock " on the high 

 ground about half a mile due east of the hotel. It was a greyish 

 * Quart. Journ. Geol. Soc. 1861, p. 185. 



