386 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
the south-western centre of eruption around Corston Hill a new volcanic 
group begins and soon increases in hulk. 
A distinguishing feature of the plateaux is found in the difference 
between the lavas that were first erupted and those which followed them. 
'J'he earlier eruptions, as above remarked, were generally basic, sometimes 
highly so. Thus at Arthur Seat the thick series of lavas which form 
the eastern part of the hill have at their base several sheets of columnar 
basalt, over which come the andesites that make up the main mass of the 
erupted material. In the Calton Hill the same sequence may he observed. 
Underneath the andesites of Campbeltown comes a well-marked and persistent 
band of oliviue-dolerite. Still more basic are some portions of the earliest 
lavas of the Garleton plateau where, as already stated, rocks present them- 
selves composed mainly of olivine and augite. 
It is worthy of notice that where the lavas of a plateau diminish greatly 
in thickness oi' become imper.sistent, the lowest basic group may continue 
while the overlying andesites disappear. This feature has been already 
mentioned as well seen in the Midlothian plateau. The thick group of 
andesites in Arthur Seat and Calton Hill is not to be found in the next 
volcanic eminence, Ciaiglockhart Hill ; but the basalts witli their underlying 
tuffs continue. In the south -wester-n tract from Harper Eig to Hare 
Law in Lanarkshire, the tliin lava-band, which can be found only at 
intervals along the line of outcrop of the volcanic series for about nine 
miles, is a dolerite often highly slaggy in structure. Again, at Corrie in 
Arran, the lavas which appear upon the shore, apparently at the extreme 
western limits of the Clyde plateau, are basic rocks. 
But whether or not the lowest and more basic lavas appear in 
any plateau, the main mass of the molten material erupted has usually 
consisted of varieties of andesite. The successive discharges of these inter- 
mediate lavas have bowed out in sheets, some of which must have been 
little more than heaps of clinkers and scoriae, while others were more fluid 
and rolled along with a ropy or slaggy surface. Occasionally the upper part 
of an andesite shows the reddened and decomposed character that suggests 
some degree of disintegration or weathering before the next lava-stream buried 
it. The intervals between successive outflows of these lavas are not, as a rule, 
defined by any marked breaks or by the intercalation of other material. In 
general, the plateaux are mainly built up of successive sheets of lava which 
have followed each other at intervals sufficiently short to prevent the ac- 
cumulation of much detritus between them. Thus the Campsie Hills have 
the upper 600 feet of their mass formed of admirably- well-defined sheets of 
andesite, separated sometimes by thin partings of tuff, hut more usually only 
by the slaggy vesicular surfaces between successive flows. 
W'here the lavas consisted of trachytes they were apt to assume more 
irregular forms. Of this tendency the rocks of the Garleton Hills supply 
an excellent example. As already stated, their lumpy character gives to 
these hills an outline which offers strong contrast to the ordinary sym- 
metrical terraced contours of the andesitic plateaux. 
