CHAP. XXV 
PLATEAU-VENTS IN SCOTLAND 
397 
view in Fig. 123 } Where two vents have been successively opened close to 
each other, or where the same vent has shifted its position, the groiind-plan 
may be greatly modified. I n some instances the double funnel can he distinctly 
traced. Thus in the conspicuous Knock Hill above T/args in Ayrshire (kig. 
125, B) there are two conjoined necks, and such api)ears to he also the 
structure shown by the ground-plan of the neck of Barwood Hill and 
Haven’s Craig, east of Dumbarton (Fig. 125, A).-^ But more complex 
forms occur which point to a still larger number of coalescing necks. 
A group of hills to the east of Dumbarton gives the ground-plan shown 
in Fig. 125, where traces may he detected of three separate vents. 
Still more irregular are long narrow dyke-like masses of tuff or agglomerate 
which have probably risen along lines of fissure (Fig. 22, Ko. 1). The most 
striking example of these, however, occur in association with the puys and 
will be described in later pages. 
Connected vdth their ground-plan is the relative size of the plateau- 
vents. On the whole they are larger than those of the puy series. The 
simple circrrlar or elliptical type presents the smallest necks, some of them 
not exceeding 100 feet in diameter. The more complex forms are generally 
also of larger dimensions. By much the largest vent or connected group 
of vents is that which lies among the uplands of Misty Law in the heart 
of the Eenfrewshire part of the Clyde plateau, where a connected mass of 
tuff and agglomerate now occupies a space of about 4 mdes in length 
by 24 miles in breadth (Fig. 129). It has not been found possible, how- 
ever to trace the boundaries of the separate vents of this tract, nor to dis- 
tinc^uish the material of the necks from that which surrounds them. 
Another large mass which from its shape may be conjectured to represent 
more than one vent is the great tract north of Melrose, wliich measures 
8800 by 4200 feet.^ 
Tlie distribution of the necks can best be understood from the maps of 
the Geological Survey, where they have lieeii carefully indicated. As might 
have been expected, they are not found outside the original limits within 
which it may be reasonably inferred that the lavas and tuffs were erupted. 
They occur most abundantly and attain their largest size in and around the 
districts where the plateaux are most extensively developed. No doubt a 
1 Tlie illustrations in Figs. m4 and 125 are taken from the field-maps of the Geological Survey on 
the scale of 6 inches to a mile. The ground represented in Fig. 124 was ma],ped by Mr. R. L. Jack^ 
^ These groiiiid-plans are likewise taken from the field-maps of the Geological Survey A and 
,, . u n.,, Mv liicl- Rhvmvself. The shaded parts are intrusive andesites and dolcrites ; 
Vk- .1 .I.- .... TU. 
i", “« f !; ““ 1 ““ 
Geoloaioal Survey. Carewood Rig, on the borders of Ro.vburghshire and Dumfriesshire, 
7000x2400 feet-^the great vent in the middle of the Campsie Fells, 5200x2600; Black Law, 
between Bedriile’ and Jedburgh, 3400 x 1600 ; Dumgoyn. Stmthblane 2300 x 1800 ; RuW^^ 
1500x1000; Minto Hill (south), 2300 x 1650 ; Miiito Hill (north), 1500 x 1100 ; Doiighnot Hill, 
Kilpatrick range, 1000x700; four of the sniallcst agg omerate ventA a ong the northern escarp- 
ment of the Clyde plateau between Strathblane and Inntry, 500x4.50 450x400, ^50 x 100, 
200x200 • Pike Law, Arkleton, Tarras Water, 500 x 500 ; Harwood, Stonedgc, 5 miles S.L. from 
Hawick, 500 x 300 ; Arkleton Burn, Dumfriesshire, 400 x 100 ; Dalbate Burn, 250 x 120. 
