OF THE TRAP-ROCKS OF SCOTLAND. 639 
consecutive series, are overlaid unconformably by the Carboniferous Limestone 
series, and that the same unconformity extends eastward between the Upper Old 
Red Sandstone and the lower member of that formation. The Carboniferous and 
Upper Old Red Sandstones form, therefore, one great physical series, and the Lower 
Old Red Sandstone and the Upper Silurian form another, while between the two 
conjoint groups there occurs a marked physical break. This new fact in Scottish 
geology is fraught with interest when viewed in connection with the intrusive 
rocks of the Silurian region. 
The Silurian shales and Lower Old Red Sandstones of Lesmahagow are traversed 
by numerous dykes and masses of porphyritic felstone. These do not extend 
into the overlying Carboniferous strata, but are covered unconformably by them. 
They must therefore be later than the Lower Old Red Sandstone, and earlier than 
the Carboniferous Limestone. There can be little doubt that these igneous rocks, 
though probably not themselves the agents in tilting and shattering the strata 
among which they occur, nevertheless were ejected during a period of movement 
and disturbance. The structure of the Pentland Hills showed me very clearly 
that the great tilting of the Lower Old Red Sandstone in South Scotland had 
taken place before the deposition of the upper member of that formation; and 
the inference is probably not too hasty that the Lesmahagow felstones may have 
been produced during this period of subterranean movement, and that they must 
consequently be of an age intermediate between the Lower and the Upper Old 
Red Sandstone.* 
From this region as a base line, and with this definite date, the igneous geo- 
logy of the southern part of the island may perhaps be eventually worked out. 
We have here the clue to the history of that great series of felspathic rocks which 
traverse the higher part of Lanarkshire, and rise into such bold hills as Tinto. 
They are all probably of Lower or Middle Old Red Sandstone age, and it is not 
improbable that intercalated sheets of ash and sandstone may occur, especially 
towards the north-east, linking in this way the intrusive felstones of Lanark and 
Ayrshire, with the Upper Old Red interbedded sheets of the Pentland Hills. 
To return to the Lammermuir region. The examination of the geology of Les- 
mahagow, though revealing many new facts in the igneous history of the country, 
did not directly throw any light on the date of the Lammermuir dykes. The evi- 
dence at present is briefly this:—We find the south of Scotland traversed by vast 
numbers of felspathic masses; some of these are of Old Red Sandstone age, and 
were thrown up during a tilting of the Lower Old Red Sandstone and older beds; 
others may possibly be older still, and may mark another period of subterranean 
movement anterior to the Upper Silurian. The district which promises best to 
* How far the Middle Old Red is represented in the great series of purplish sandstones of Lanark- 
shire cannot at present be determined. It would probably be nearest the truth, however, to regard 
these felspathic dykes as of Middle Old Red Sandstone age. 
