644 MR ARCHIBALD GEIKIE ON THE CHRONOLOGY 
The upper part of the Old Red is consequently to a marked extent felspathic; the 
conglomerate there consists largely of various felstone fragments broken from the 
subjacent beds, and the grits in like manner display a considerable quantity of fels- 
pathic matter as a cementing paste. The felstone beds which follow vary greatly 
in texture and composition. Sometimes they are dark and crystalline, or pale and 
dull in texture, or earthy and highly amygdaloidal. Throughout the central and 
southern part of the range, these felstones are well defined, each sheet being 
traceable for a considerable distance,—sometimes for six or eight miles. Between 
these beds there occur thinner interstratifications of felspathic ash, sandstone, 
conglomerate, and breccia. At the north end of the chain much confusion super- 
venes, owing to the multitude of minor lenticular beds which occupy the long 
parallel sheets that run southward. Beds of felstone of all shades and tex- 
tures, porphyritic, compact, and amygdaloidal, follow each other, or are inter- 
blended together with no apparent order, further than that their general line of 
strike corresponds with that of the central and south part of the hills. The 
bedded character of these masses is rendered more apparent by the occasional 
interstratification of ashy bands. These, however, are also of limited extent. 
In short, the whole aspect of the north end of the chain renders the conjecture of 
Mr M‘Laren* a very probable one, that here or hereabouts lay the centre of 
eruption round which the minor flows accumulated, while the larger lava streams 
flowed for miles to the south-west, covering over or lapping round the islets of 
slate which had not been enveloped by the grits and conglomerates of the Old 
Red group. These igneous rocks are covered by conglomerates usually more or 
less felspathic, and these again gradually pass up into the finer calcareous con- 
glomerates, sandstones, and shales of the Lower Carboniferous group. 
In short, the Pentland Hills afford evidence that, at the time of the Upper 
Old Red Sandstone, the district to the south-west of Edinburgh was for a long 
while the seat of a powerful volcano, which sent out massive streams of lava and 
showers of ash, and continued active until well-nigh the dawn of the Car- 
boniferous period. It will be the work of future years to ascertain the areas 
in Scotland which formed contemporaneous centres of volcanic eruption. 
Trap-Locks of Carboniferous Age. 
We pass now to the Carboniferous system, which contains a more abundant 
series of the volcanic rocks than any other Scottish formation. The whole of 
the broad Carboniferous band which bisects the country from the Firth of Clyde 
to the mouth of the Forth and of the Eden is intersected in every direction by 
dykes and masses of trap. Nor is it merely their number that renders these 
igneous rocks remarkable. They take a large variety of forms, occurring as intru- 
* Geol. of Fife and Lothians, p. 183. In this volume the structure of the Pentland Hills is 
delineated with great clearness, and in a truly philosophical spirit. 

