104 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



occupying the hollow must be softer than that of the ring is obvious, for great as is the 

 temptation to look on this as a crater-lake, we must admit that what we now see is not 

 the original surface, but has been exposed after the removal of possibly hundreds of feet 

 of overlying material. The present lake-basin, whether or not it may represent a former 

 crarer-lake, is undoubtedly due to erosion. Possibly some more easily removable 

 agglomerate may here occupy the centre of the volcanic pipe. 



One of the most interesting features of this vent is to be found in its relation to the 

 surrounding basalts. The marginal parts of the rock along the line of contact are much 

 finer in grain than the rest, and have obviously cooled more rapidly. The contrast 

 between them and the ordinary dolerite of the centre, however, cannot be properly 

 understood, except in thin sections under the microscope. Dr Hatch observes that, in 

 place of the structure above described, the marginal parts show an absence of the ophitic 

 grouping except in small isolated patches. Instead of occurring in large grains or plates 

 enveloping the felspars, the augite is found in numerous small roundish grains, together 

 with grains of magnetite, in equal abundance and of similar size. The felspars are 

 spreckled over with opaque particles ; olivine was not detected. 



For miles around the vent, the plateau-rocks are of the usual type — black, compact, 

 sometimes amygdaloidal, alternating with more coarsely crystalline decomposing bands, 

 the separation between different sheets being often marked by the ordinary red ferruginous 

 partings. But around the margin of the neck, they have undergone a remarkable meta- 

 morphism. The portions of them which adhere to the outer wall of the neck have lost 

 their distinct bedding, and have been, as it were, welded together into an indurated 

 compact, black to dull-grey rock, so shattery and jointed that fresh hand-specimens, 

 three or four inches in length, are not easily obtainable. Especially marked is one set of 

 joints which, running approximately parallel, cause the rock to split into plates or slabs. 

 These joints are sometimes curved. . Yet, in spite of the alteration from its normal 

 character, the basalt retains in places some of its more usual external features, such, for 

 instance, as its amygdaloidal structure, the amygdules consisting of calcite, finely acicular 

 mesotype, and other minerals. 



Examined under the microscope, this altered basalt presents " a confused aggregate of 

 colourless microlites (felspar V) and innumerable minute granules of magnetite, these two 

 constituents being very unequally distributed. Sometimes the colourless portions pre- 

 ponderate, in other places the opaque granules are heaped together in black patches, 

 which may possibly mark the position of fused augites " (Dr Hatch). 



In the zone of contact-metamorphism around some of the volcanic pipes in the 

 plateaux, we see changes analogous to, but less developed than, those which have been 

 superinduced on so large a scale round the great eruptive bosses of gabbro, granophyre, 

 &c, that have broken up the terraced basalts along the west coast of Scotland. I shall 

 accordingly return to this subject in connection with the phenomena presented by these 

 younger rocks. 



(b) Vents filled ivith Agglomerate. — Though much less frequent than the necks of 



