604 MESSES. CLOT7GH, MAUEE, ANE BAILEY 05T [Nov. I9O9, 



Movement acting before, after, or during the period of intrusion 

 appears to be the only possible cause of asymmetry admitted by 

 the conditions of the problem. 



Movement before the intrusion might lead to asymmetry by 

 faulting down a relatively impenetrable type of rock on the inside 

 of the cauldron-subsidence against a penetrable type on the outside. 

 If, for instance, the whole of the foundered area were occupied, at 

 the present surface, by lavas thrown against the schists outside, this 

 explanation would appear extremely reasonable. But, as a matter 

 of fact, the lavas have been so far removed by erosion that along 

 half its course the boundary-fault is seen separating schist from 

 schist. More than this, it is possible to recognize inside the cauldron 

 the displaced equivalents of the very same members of the schist 

 succession as occur outside. Tbe asymmetrical distribution of the 

 fault-intrusion is, therefore, not determined by any peculiar character 

 of the down-faulted rocks : and accordingly it cannot be due to 

 movement antedating the intrusion. 



Faulting after intrusion might seem, at first sight, to account 

 for the peculiarities of this distribution. Thus, if a consolidated 

 igneous mass were to sink, roof and all, to a lower level, leaving 

 merely its outer rim standing to mark its former presence, then 

 denudation would eventually reveal a complex with much the same 

 general appearance as that of Glen Coe. But faulting after the act 

 is ruled out of court, in this case, for in a host of exposures the 

 fault-intrusion is clearly chilled against the fault-plane. In some 

 of these sections it is firmly soldered on to the fault-rock, having 

 suffered no movement whatsoever since consolidation ; and even in 

 the more numerous instances where the junction has been shattered 

 by later tremors following the old line of weakness, it has been so 

 little moved that the thin chilled margin of the intrusion has not 

 been displaced. Obviously, then, movement after the act cannot 

 have determined the distribution of the fault-intrusion. 



We are thus led to regard the faulting and the intrusion 

 as contemporaneous events. They represent two aspects of a 

 single adjustment. The magma welled up around the subsiding 

 mass, like the liquor in a full bottle when the stopper settles home. 

 The slowly rising stream found in the fault-plane its easiest avenue 

 of escape. Thence, we may suppose, it tended to spread out 

 symmetrically into the rocks on either side ; but any portion which 

 trespassed into the sinking mass was by a continuation of the 

 subsidence carried down. Thus contemporaneous movement along 

 the fault supplies the element of asymmetry which is the essence 

 of the problem. It delayed the entrance of the intrusion into the 

 downthrown mass, in comparison with the zone lying immediately 

 outside. Within limits, this effect would certainly be cumulative, 

 since each fresh invasion of the outer zone furnished an additional 

 channel for the upward and outward movement of the ascending 

 magma. 



A surprising feature of the Glen Coe phenomena is the frequency 



