446 
THE CARBONIFEROUS VOLCANOES 
BOOK VI 
iferous times. Xotwitlistaiiding their volcanic nature, they can he traced 
and mapped precisely as if they had been limestones or sandstones. This 
■jjerfect conformahility with the associated stratified rocks is strikingly seen 
in tlie case of the sheets of lava which lie imbedded in the heart of the 
great rolcanic ridge ol Linlithgowshire. The overlying strata having lieen 
Fiu. 157.— Section from Linlithgow Loeli to the Firth of Forth. 
ieino\''ed from their surtace tor some distance, and the ground having been 
broken by faults, these volcanic rocks might at first he taken for irregular 
intrusive bosses, but their true character is that shown in Fig. 157, where 
by a succession of faults, with a throw in the same direction, the up2ier 
basalts of ];>onnytoun Hill are graduiilly brought down to the level of the 
Firth of Forth. 
iii. SILLS, BOSSES AND DYKES 
One of the characteristic features of Centrid Scotland is the great 
number, and often the large size and extraordinary persistence, of the masses 
ot eruptive, more or less basic material, which have been injected among the 
t. arbonilerous stratii. The Jirecise geological age of these intrusions cannot, 
ot course, be more exactly defined than by stating that they are younger 
than the rocks wdiich they traverse, though in many cases their association 
with the necks, lavas and tuffs is such as to show that they must be 
regiircled as part of the Carboniferous volcanic phenomena. 
Sills. — With regard to the sills I have been led, for the following 
reasons, to connect the great majority of them with the puys, though some 
aie certainly of far later date, while others should possibly be assigned 
to the jdateaux. 
In the first place, the sills obviously connected with the plateaux are in 
great measure intermediate, or even somewdiat acid roclcs, wdiile those of the 
puy sei'ies are much more basic. It is hardly possible, however, in all cases 
to decide to which series a iiarticular sill should be assigned. This difficulty 
is particularly manifest in the western jiai’t of Midlothian, where the plateau 
ot that district exhibits such frequent interru^ition, and where it often con- 
sists only of a single basaltic sheet. To the west of it lie the abundant 
puys wdth their lavas and tuffs, and between the two volcanic areas 
numerous sills of dolerite and diabase make their ap2)earaiice. In the 
difiiculty of deciding to which series these sills should be referred, it wall be 
convenient to consider them with those of the puys. 
