Vol. 65.] THE CAULDRON-SUBSIDENCE OE GLEN COE. 641 



and usually a little quartz. The rest of the dykes comprise quartz- 

 porphyries, felspar-porphyries, and several varieties of lamprophyre. 

 There is, thus, a considerable range in composition, but the various 

 rock-types form a connected suite without any marked break. This 

 bespeaks consanguinity, and so, too, does the close association of 

 the various rock-types in one dyke-belt, and even, as is frequently 

 the case, in one composite dyke. Besides this, the assemblage of 

 types can be matched in the dykes following many other well-known 

 granodioritic intrusions, both British and foreign. 



Mr. Kynaston has shown that, although numberless dykes enter 

 the Cruachan Granite, ' only one small porphyrite dyke ' has been 

 observed in the whole extent of the Starav Granite. He supposes, 

 therefore, 1 that the dyke-phase of intrusion may have intervened 

 between the uprise of the two plutonic masses. Our own observa- 

 tions certainly favour this interpretation, since a specimen from a 

 porphyrite dyke cutting the Cruachan Granite near the margin of the 

 Starav mass shows strong evidence of contact-alteration (13762)." 



1 ' Geology of the Country near Oban & Dalmally' Mem. Geol. Surv. 1908, 

 p. 87. 



2 [Since the reading of this paper we have taken an opportunity to study the 

 phenomena more fully in the field, and have found that the Starav granite 

 is clearly later than the great suite of north-north-east dykes. The junction 

 of the two granites is exposed for over a quarter of a mile in Allt nan Gaoirean, 

 the southernmost tributary on the right bank of the River Etive shown on the 

 map (PI. XXXIV). In this burn the Cruachan Granite is cut by dykes, while 

 the Starav Granite is not. The dykes show distinct signs of contact-alteration 

 in the hand-specimen, and microscopic examination of thin slices confirms it 

 (14175, 14177-78). The junction-plane of the granites is here a fault, which is 

 accompanied by shearing along the junction, and also along subsidiary parallel 

 shear-planes traversing both Cruachan Granite and dykes alike. The section 

 is thus less decisive than would otherwise have been the case, but at the same 

 time the faulting and shearing are of interest in another connexion. 



Allt Dochard, flowing obliquely across the junction-plane a little south of the 

 area included in PI. XXX IV, furnishes corroborative evidence. Dykes are 

 here abundant in the Cruachan Granite quite near to the Starav margin, while 

 they are absent in the latter intrusion where it in turn forms the bed of the 

 stream. Although they consolidated with typical hypabyssal structure, these 

 dykes are traversed, in common with the Cruachan Granite, by numerous aplite 

 veins. Examined under the microscope, thin slides of some of them show 

 evident contact-alteration (14181, 14182, & 14183). 



A north-north-east lamprophyre dyke cuts across the Starav Granite-junction 

 and is chilled against both granites in the bed of the River Kinglass. It differs 

 petrologically from the dykes of the great north-north-east suite, in containing 

 purple augite and in other characters (14189). This is the only dyke that 

 can correspond to Mr. Kynaston's unique example of a porphyrite dyke cutting 

 the Starav mass. A ' porphyrite ' dyke has indeed been mapped farther up 

 Glen Kinglass within the Starav Granite, but this is really a pegmatite. The 

 few dykes found in the heart of the Starav mass trend north-west and south- 

 east, and are petrologically quite distinct from the rocks under description 

 (for instance, 7762 ie a camptonite). Thus, not a single normal member of the 

 north-north-east suite of dykes cuts . the Starav Granite, while a considerable 

 number have undergone contact-alteration in its immediate neighbourhood. 



Special visits were paid to the sections on Beinn Sguliaird and in Glen 

 Dochard, quoted by Mr. Kynaston as showing a passage for a short space 

 between the Cruachan and Starav Granites. In the clearest sections here we 

 found a sharp line between the two intrusions, but in weathered, lichen- and 

 moss-covered crags it is frequently difficult to locate the exact junction. 



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