Yol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 673 



The rock constituting this long strip is marked in one locality by 

 thin nearly vertical seams, in which it occurs in a finely sheared 

 granulitic condition (11498). The strike of these grauulitic seams 

 is almost the same as that of the band of granite of which they 

 are a part. Certain uncrushed strings belouging to this band also 

 occupy lines of fault, which displace to a small extent thin fine- 

 grained aplite strings belonging to the Cruachau Granite. The 

 inference from these observations is that before and after, and 

 perhaps even during, the marginal uprise of the Meall Odhar 

 intrusion, there was a tendency for the southern portion of the 

 Cruachan Granite to sink along its original northern boundary. 

 This movement was not of great extent, and was discontinued when 

 the conditions of the dyke-phase reasserted their sway. But at a 

 later period, as we have seen reason to believe, the downward 

 movement was resumed, during the intrusion of the Starav Granite, 

 although now it merely affected the central portion of the boss. 



In putting forward this interpretation of one particular plutonic 

 mass, we do not wish to insist upon the essential passivity of intru- 

 sive magmas in general. The hypothesis which we have advanced 

 is in keeping with the views of Suess, and has much in common 

 with the piecemeal stoping lately advocated by Daly, a hypothesis 

 which we have employed ourselves to account for certain features 

 of the fault-intrusion of Glen Coe. On the other hand, large 

 intrusive masses, entering horizontally stratified formations, are 

 well known to have a marked tendency to assume a laccolitic form ; 

 while other manifestations of the disruptive force of magma under 

 pressure might readily be enumerated. It is obvious that any 

 conception of the form of an intrusive mass must be of the nature 

 of an ideal, approached more or less closely, perhaps, but never 

 actually realized, in nature. It must not be imagined, for instance, 

 that we regard fig. 14 (p. 670) as anything more than diagrammatic. 

 In particular we believe that the sinking plugs of solid rock beneath 

 the various cauldrons must become more and more broken up and 

 perhaps dissolved at increasingly lower levels, so that eventually 

 great tubes are formed, consisting largely of molten material. 



(d) The dykes of Etive. — Two main features call for inter- 

 pretation in regard to the north-north-easterly porphyrite dykes 

 of the district ; first, their regional constancy in direction, and 

 secondly their local concentration with reference to the Etive 

 centre. 



The regional constancy in direction of the dykes undoubtedly 

 bespeaks a distribution of earth- stresses different from that which 

 prevailed during the production of either the Glen Coe cauldron or 

 the Etive boss, although possibly in the north-north-eastward 

 elongation of the latter and in its alignment with the Glen Coe 

 cauldron there may be a first suggestion of that orientation of 

 earth-stresses which culminated during the dyke-phase. 



We have shown that the introduction of the dvkes has been 



2z2 



