CHAP. XLI 
VEJVTS OF THE BASALT-PLATEAUX 
285 
fragments represent most of the varieties of the lavas of the district. Laige 
slaggy masses are abundant among them, and sometimes exhibit the 
anneli de-like elongation of the vesicles which I have referred to as occasion- 
ally displayed by the plateau-basalts. More than 60 feet of agglomerate 
are visible in vertical height from where its base is concealed by debris and 
vegetation to where its upper surface passes under a banded rock to be 
afterwards described. That this unstratified mass of volcanic detritus marks 
the site of a vent can hardly be doubted, although denudation has not 
revealed the actual walls of the chimney. The steep grassy slopes do not 
permit the relations of the rocks to be everywhere seen, but the agglomerate 
appears to pass laterally into finer, rudely-stratified material of a similar 
kind, which extends towards east and west as a thick deposit between the 
bedded basalts. Possibly denudation has only advanced far enough to lay 
bare the crater and its surrounding sheets of fragmentary material, while the 
chimney lies still buried 'underneath. 
To the east or left of the agglomerate the detritus becomes less coarse, and 
shows increasing indications of a bedded arrangement. Close to the agglo- 
merate the dip of the coarse tuff is towards that rock at about 10 . A few 
yards further east a sheet of very slaggy basalt is seen to lie against the 
tuff which it does not pierce. The vesicles in this adhering cake of lava 
have been pulled out in the direction of the slope till they have become 
narrow tubes four or five inches long and parallel to each other. Some paits 
of this rock have a curved ropy surface, like that of well-known Vesuvian 
lavas, suggestive of the molten rock having flowed in successive thin viscous 
sheets down the slope, which has a declivity of about 30°. This part of the 
section may possibly preserve a fragment ot the actual inner slope ol the 
crater formed of rudely-bedded tuffs. 
Continuing still eastward, we find the feebly stratified tuff ( a ) to be perhaps 
200 feet thick. It forms a grassy declivity that descends from the basalt- 
escarpment above to the grass-covered platform which overlies a lower group 
of basalts. The visible portion of this tuff presents a thoroughly volcanic 
character, being made up of the usual dull dirty- green granular paste, 
through which are dispersed angular and rough lumps of slag and pieces of 
.more solid basalt, varying up to a foot or two feet in length. these stones 
are generally disposed parallel to the indistinct bedding, but are sometimes 
placed on end, as if they had. assumed that position on falling from an 
explosive shower. Among the smaller stones, pieces of a finely vesicular 
basic pumice are frequent and are among the most strikingly volcanic 
products of the deposit. From a characteristic sample of these stones, a 
thin slice was prepared and placed in Mr. Marker's hands. 1 he following 
are his observations on it : — “ A very compact dark grey rock, amygdaloidal 
on a minute scale. The lighter grey crust is probably due merely to 
weathering, and the specimen seems to be a distinct fragment, not a true 
bomb. The slice shows it to be essentially a brown glass with only 
occasional microscopic crystals of a basic plagioclase. It has been highly 
vesicular, and the vesicles are now filled by various secondary products, 
