■ROCKS OF THE NOETHEEN HIGEIAtfDS. 375 



in the " Logan Eock." The epidosite and the nodules of horn- 

 blende are surely exceptions to this. Assuming, however, the 

 general accuracy of Dr. Heddle's generalization, I may observe as 

 follows : — 



(1) That the occurrence of accessory minerals in some parts of a 

 formation does not prove that they will be found throughout its 

 entire extent. The Caledonian, for example, usually contains few 

 minerals, but at Shiness they are found in rich abundance. 



(2) That the western area is in some places penetrated by 

 granite veins. May not these have had something to do with the 

 accessory minerals? Indeed, most of the minerals identified by 

 Dr. Heddle are referred by him to the Cape Wrath-Durness dis- 

 trict, in which granite veins abound, and some of the minerals are 

 stated to occur in intrusive dykes, a fact which I myself observed 

 near Durness. 



(3) That the area occupied by the " Logan Eock " is insignificant 

 compared with the large tract of Hebridean; and, from what I 

 have seen of the old gneiss, I more than suspect that many areas 

 of undoubted Hebridean could be found in which minerals are as 

 scarce as in the " Logan Eock." The only minerals (omitting the 

 epidosite and hornblende nodules) I have detested away from 

 Durness are some nests of brown garnet, found above Kyle Sku ferry. 



While, therefore, conceding that Dr. Heddle's objection deserves 

 consideration, I cannot admit that it is at all decisive of the ques- 

 tion. General characters are of much more value than local pecu- 

 liarities ; and in this case the general resemblances are clear and 

 decided. 



The comparative absence in the "Logan Eock" of planes of 

 separation between beds has been adduced as another mark of 

 difference. Here, however, I dispute the alleged fact entirely. 

 Although familiar with both rocks, I have been unable to detect 

 any marked distinction between them in this respect. 



(4) A difference of weathering has also been asserted. The He- 

 bridean is said to be more thickly clothed with vegetation than the 

 " Logan," owing, presumably, to the formation of more soil. But I 

 believe this is due merely to a difference of level. The "Logan" gene- 

 rally forms much higher ground, and has been more wideh T cleared of 

 its Boulder-clay. At low levels it is as green as the Hebridean. 

 On the other hand, on lofty ground, as on the higher slopes of Ben 

 Stack, the Hebridean has the bare " knobby " appearance of the 

 " Logan Eock," 



b. The Author's Objections to the Contemporaneity of the " Logan 

 Rock " with the Assynt Series. 



It is granted at starting that the " Logan Eock " does actually 

 overlie some part of the Assynt group ; but the contention is that 

 this position is due, not to deposit, but to dislocation. 



1. The "Logan RocJc" not conformable to the roclcs below it. — 

 I have already shown that at Ullapool the massive gneiss rests at 

 different points upon each of the members of the Assynt series, 



2e2 



