396 W. M. Sudleston — First Impressions of Assynt. 



is seen to pass under tlie " Logan " rock, the angle of dip being about 

 28°. It is true that the beds in immediate contact are not very 

 typical representatives of " Logan " rock, but the great gneissic 

 masses very soon come on in force. Further eastwards the line of 

 the dolomitic series may be traced in almost vertical masses across 

 the head of the Loch. Geologists can hardly draw any absolute 

 conclusions from this place alone ; but whilst some see in the above 

 indications a conformable upward sequence, others will suspect the 

 presence of an inversion. In its general effects this latter would 

 produce almost the same results as if the Quartzite-dolomite merely 

 abutted against the " Logan " rock in the way supposed by Nicol. 



What is the " Logan " rock petrologically ? If we are to take 

 this very district of Glen Coul, we find plenty of red orthoclase, of 

 quartz and felspar, and of hornblende, sometimes in foliations, some- 

 times massive. Other varieties also occur, but one of the most diffi- 

 cult tasks is to make out any definite system of divisional planes, 

 which might be regarded as showing the bedding, so as to determine 

 the dip and strike. I hold in my hand at the present moment a 

 thoroughly typical specimen of " Logan" rock obtained from the 

 somewhat isolated exposure to the west of Ledbeg. It is striped 

 something like a tiger, with bands of a hackly pinkish felspar, 

 partially relieved by dull white quartz, alternating with thick or 

 thin bands of a dark-green matted hornblende. The cracks and 

 backings are lined with abundance of pale green epidosite, which is 

 so characteristic of the rock in the Logan valley. Altogether this is 

 a fair specimen, though there are others far richer in quartz. I 

 have usually been disposed to regard the abundance of epidosite as 

 one of the features of the '' Logan" rock which especially distinguish 

 it from the Hebridian Gneiss, but Dr. Heddle states ^ that this peculiar 

 melange occurs in greater quantity and perhaps in a purer state in 

 the west of Eoss-shire than in any locality known to him. This is 

 in the Hebridian Gneiss near Poolewe. Hence this presumed dis- 

 tinction vanishes. 



Although, therefore, there are certain differences in the aspect of 

 hand specimens, and still more perhaps in the general character and 

 behaviour of large masses, yet the " Logan " rock appears to me to 

 possess more resemblance to the Hebridian Gneiss than to any other 

 formation in the district. There must be differences, however ; other- 

 wise so acute an observer as Nicol would hardly have continued to 

 call this rock " granulite," " syenite," " diorite," according to the 

 district he was describing. In Assynt his " Logan " rock is mostly 

 " syenite," and he appears to have regarded it as intrusive. It is 

 probably owing to the circumstances connected with its appearance 

 in the position now occupied by it in the Assynt mountains, and 

 largely also to partial injection by local extravasations and to crush- 

 ing during folding, that the main points, in which the " Logan " 

 rock differs from ordinary Hebridian Gneiss, arise. And yet I 

 hesitate whilst writing this, bearing in mind that Dr. Heddle, who 

 knows both rocks so well, does not perceive the relation, which 

 1 Min. Mag. No. 20, p. 211. . 



