402 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
hill or ridge consists partly of a coarse agglomerate, and partly of veins and 
irregular protrusions of a dark, compact, slightly cellular lava. The stones 
in the fragmental rock are different olivine-hasalts, or other basic lavas, and 
sandstones. The paste is rough, loose and granular. The sandstone frag- 
ments are much indurated and sometimes bleached. 
The Knock itself is foimed mainly of a remarkably coarse and strikingly 
volcanic agglomerate. Kound the outside, and particularly on the south-east, 
the rock is finer in texture, compact, and gravelly, or like a mudstone, with 
few or no imbedded blocks, dull-green to red in colour, and breaking with 
a clean fracture which shows angular lapilli of various basalts or diabases. 
At the southern end of the neck, where the surrounding red sandstone can 
be seen within a few feet of the tuff, the latter is bright red in colour, and 
contains much debris of red sandstone and marl. Possibly this finer tuft’, 
which is traceable as an irregular band round the outside of the neck, may 
mark an older infilling of the vent than the agglomerate of the centre ; but 
tliere is no sharp line to be drawn between the two, though a hollow can 
sometimes be traced on the surface where they join. 
The agglomerate of this locality is one of the most characteristic among 
the plateau-necks of the Clyde region. Its blocks sometimes measure from two 
to three feet in diameter. They consist almost wdiolly of a dark crystalline 
porphyritic olivine-basalt. Tliese blocks are subangular in form, often with 
clean-fractured surfaces. fi'hough occasionally slightly cellular, tliey are 
never slaggy so far as I could see, nor are any true scoriae to be noticed 
among them. The blocks suggest that they were derived from the disrup- 
tion of an already solidified mass of lava. The agglomerate is entirely 
without any trace of stratification. 
Through this tumultuous accumulation of volcanic debris some 
irregular veins of olivine-basalt, sometimes glassy in structure, have been 
injected, and reach nearly to the summit of the hill. This intrusive 
material resembles generally some of the dark intrusive masses in the 
Dumbartonshire necks. Like these, it exhibits a tendency to assume a 
more or less distinctly columnar structure, its columns having the same 
characteristic wavy sides and irregular curvature. The intrusn^e rocks in 
the two eminences of the Knock may be paralleled among the stones in the 
agglomerate. The neck on its north-eastern side rises steeply from the red 
sandstones which it pierces, but which, although they are much jointed and 
broken, are not sensibly indurated. Unfortunately the actual junction of 
the igneous and sedimentary rocks is concealed under herbage. 
As a rule, the fragmental materials of the plateau-necks are quite 
unstratified. Their included blocks, distributed irregularly through the 
mass, have evidently undergone little or no assortment after they fell 
back into the vents. Occasionally, however, a more or less distinct 
bedding of the agglomerate or tuff may be observed, the layers having 
a tendency to dip inward into the centre. One of the most conspicuous 
examples of this structure is to be found in the hill of Dumbuck, to 
the east of Dumbarton. This neck, which forms so prominent a feature 
