ig2 
THE S/L URIAH VOLCANOES 
BOOK IV 
Owing, however, to the constant plication of the strata, and the wide space 
which the overhdng sedinientary deposits are thus made to cover, the 
volcanic group only comes occasionally into view, and thus occrrpies but a 
mere fraction of the superficial extent of the region over which its scattered 
outcrops appear. These exposures, sometimes only a few square yards in 
extent, may always be looked for where the anticlinal folds bring up a 
sufficiently low portion of the Silurian system ; they prove that a vast 
volcanic floor underlies the visible Lower Silurian grits and shales over the 
length and breadth of the Southern Uplands of Scotland. 
Without anticipating details which will projieiiy appear in the official 
Memoirs of the Geological Sm-vey, I may briefly indicate the visible boundaries 
of the volcanic group, and refer to some of the localities where it may best be 
seen. The most easterly points where it has been recognized by Mr. Peach 
stand on the crests of some sharp anticlinal folds near St. Mary’s Loch and 
near Leadburn and Winkstoue in Peeblesshire. Farther westwards it appears 
at many places along the northern border of the Silurian territory, as at 
Komanuo Bridge, Wrae, Kilbucho, Culter Water and Abington, the length and 
breadth of each exposure depending partly on the breadth of the anticline 
and partly on the depth to which it has been cut down by denudation. 
Near Sanquhar the volcanic series opens out for a breadth of more than a 
mile, and is seen at intervals across the wild moorlands of Garrick, until 
from the Stinchar valley it widens out seaward and occupies much of the 
coast-line of Ayrshire between Girvan and the mouth of Loch Piyan. It 
probably rises again along a fold near Portpatrick, and it is seen at 
various points along the southern borders of the Silurian uplands, as near 
Castle-Douglas, at Glenkiln, Bell Craig near Moffat, and the head of Ettrick- 
dale. 
The best sections are those exposed along the coast to the north and 
south of Ballantrae. When that ground was first examined by the Geo- 
logical Survey, the hypothetical views in regard to metamorphism already 
referred to were in full ascendant, and the rocks were mapped on the same 
general principles as those which had been followed in Wales. Professor 
Bonney, however, a few years later recognized the true igneous nature of 
many of the rocks. He found among them porphyrite lavas and agglomer- 
ates which he regarded as of Old Bed Sandstone age, likewise intrusive 
serpentines and gabbros.^ 
The volcanic rocks of this wide district include both lavas and their 
pyroclastic accompaniments, as well as intrusive sills and bosses of various 
materials. They have recently been studied by Mr. J. J. H. Teall, and full 
descriptions of them by him will appear in a forthcoming volume of the 
Memoirs of the Geological Survey. He has ascertained that though 
generally more or less decomposed, the lavas would be classed by German 
petrographers as diabases and dialrase-porphyrites. The former are com- 
pact dark-green non-porphyritic rocks, often containing numerous small 
spherical amygdales ; while the latter are markedly porphyritic, enclosing 
^ Quart. Journ. Geul. Soo. vol. xxxiv. (1878), p. 769. 
