50 SECTIONAL ADDRESSES. 



lateral spread of the magma and depression of its roof. This theory was 

 put forward by Mr. Bailey to explain the pronounced overfolding of the 

 margin of the north Arran granite previously referred to, but it appears 

 to be equally applicable to the arcuate folding that surrounds the south- 

 eastern caldera of Mull. The folding in the latter instance was presumably 

 caused by early intrusion of granophyre (Glas Bheinn granophyre) guided, 

 as has been suggested, by the peripheral ring-fracture that bounded the 

 area of central subsidence. 



Dr. Daly, writing two years ago on the relation of mountain building 

 to igneous action, commented upon this very case, and suggested that the 

 arcuate folding was possibly due to the centrifugal sliding of large fragments 

 of a complex dome which had become unstable because of its high elevation 

 and the weakening of the interior by magmatic injection. The difference 

 between the views of Dr. Daly and Mr. Bailey is that the former would 

 make the folding subsequent to the formation of the dome, while Mr. 

 Bailey would regard the doming and folding as practically contempo- 

 raneous phenomena dependent upon the intrusion of the igneous mass. 



Iceland, although affording good examples of ring-fractures, has not 

 so far been credited with ring-dyke intrusions. In the plutonic regions 

 of the south-east all but one of the intrusions examined by Dr. Hawkes 

 are stocks, which, like the ring-dykes of Britain, have been intruded in 

 tensionally stressed regions. Of these the granophyre stock of Slaufradal 

 is one of the most beautiful examples of its kind. It is elliptical in outbne, 

 has steeply inclined walls and an almost flat roof. It is exposed to a 

 depth of more than two thousand feet, and the almost horizontal plateau 

 basalts that constitute its walls and roof are absolutely undisturbed. 

 Such a form of intrusion is that which I consider the earliest member 

 of a ring-dyke complex is most likely to assume. 



The one exception, in Iceland, to the perfect replacement of country- 

 rock without disturbance of the surrounding strata is the plutonic mass 

 of Faskrudsf jord. In this case Dr. Hawkes informs me the basalts are 

 tilted into an almost vertical position against the intruding granophyre, 

 and there are other signs of forcible disturbance of the country-rock. 



As a generalization, however, we may state that the ring-dyke, or stock, 

 with its circular walls and flat or gently domed roof, is the type of intrusion 

 that specially characterises the plutonic centres of the Tertiary province. 

 It should certainly be borne in mind when studying the form of intrusions 

 that a simulation of laccolitic intrusion can be produced, as we have 

 seen, by the lateral spread of magma in the upper portion of a domed 

 intrusion of ring-dyke type. 



One of the most important discoveries of Dr. Harker in Skye was his 

 detection of a great group of basic sheet-like intrusions that were inclined 

 inwards towards a definite centre. He demonstrated that their distribu- 

 tion and their regular inclination towards the centre of the great gabbro 

 intrusion of the Cuillins proved them to be a local group connected 

 with the gabbro-centre, though much younger than the plutonic rocks 

 themselves. 



When Mull and Ardnamurchan were surveyed it was found that inclined 

 sheets, or cone-sheets as they have been synonymously called by the 

 Survey, played even a more important part than they did in Skye. 



