CHAP. XIX 
MONTROSE CENTRE OF ERUPTION 
299 
The distribution of these various volcanic areas will be most easily 
understood from an examination oi Map III. accompanying this volume. 
I. THE NORTHERN CHAIN OF VOLCANOES IN “LAKE CALEDONIA” 
1. The Montrose Centre 
Beginning at the north-eastern end of the area, we first encountei’ a 
series of volcanic rocks which attain their maximum thickness in Forfarshire 
around the town of Montrose. The main vents probably lay somewhere to the 
east of the present coast, uirder the floor of the North Sea; at least no clear 
indication of their existence either on the coast or inland has beerr detected. 
From Montrose, both to the north-east and south-w'est, the lavas thin awaj , 
becoming irrtercalated among the sandstones, flagstones and conglomerates, 
and gradually dying out. The total length of the v^olcarric belt is about 18 
miles, that is nine nriles from the ceirtral thick irrass irr a irorth-easterly arrd 
the same distance in a south-westerly direction.^ The volcanic prle must be 
several thousand feet thick, but owiirg to the prolongatiorr of the great Ochil 
anticlirre, the lavas roll over and do not allow their base to be seen. The 
axis of the fold nrust pass out to sea, through the hollow on vyhich the town 
of Montrose stands. The volcanic series consists of andesite-sheets with 
volcanic conglomerates. It contains little ordinary tuff, but the conglomer- 
ates no doubt partly represent ejected fragmental material, as well as the 
waste of exposed lavas. A section across the anticlinal fold from Forfar to 
Panbride, a little to the south-west of Montrose, would reveal the structure 
shown in Fig. 67. 
In the north-eastern prolongation of the volcanic series from the 
Montrose centre, successively lower members are exposed along the coast- 
line. But the lavas are dying out in that direction, and sometimes many 
hundreds of feet of ordinary sediment intervene between two successive 
flows. It was in one of these long pauses near the top of the whole 
pile of lavas that the strata of Canterland were deposited, to which reference 
has already been made. South-west from Montrose the thick volcanic 
mass rapidly diminishes, and is prolonged to the end only by three or foiii 
bands separated by sandstones and flagstones. It is in these intercalated 
groups of sedimentary material that the “ Forfarshire flags occur. 
Nowhere can the details of the Old Bed Sandstone volcanic rocks be 
more conveniently studied than along the coast-section in this district fiom 
the Bed Head to Stonehaven. The rocks have not only been cut into 
vertical clifls, but along many parts of the shore they have been also laid bare 
in ground-plan, so that a complete dissection of them is presented to the 
geologist. At the south end, the top of the volcanic series appears at the 
1 The south- western part of this area from Arbroath to Johushaven was mapped for the 
Geological Survey by the late Mr. H. M. Skae, the uorth-eastern part by Mr. D. R. Irvine. My 
account of it is mainly taken from notes made by myself on the ground preliminary to the 
commencement of the mapping of the Survey. 
