P. Macnair # J. Re id — On the Old Red of Scotland. Ill 



be little doubt, as the older geologists believed, that the whole of 

 the Highlands of Scotland were at one time covered with a vast 

 thickness of Old Red Sandstone rocks. The period at which this 

 denuding process must have begun would of course be subsequent 

 to the metamorphism of the lower Palaeozoic rocks themselves, and 

 was probably contemporaneous with the deposition of the higher 

 members of the Upper Silurian age. That this is so is evident from 

 the manner in which the latter pass up conformably into the lower 

 Old Red Sandstone at Leshmahagow, in Lanarkshire. They there 

 contain an abundant supply of Upper Silurian forms, and un- 

 doubtedly go to prove that the work of carving out the great chain 

 of upheaved and metamorphosed older Palaeozoic rocks had already 

 commenced along the skirts of the chain. These deposits seem to 

 us to represent somewhat of a deep-water aspect, and it is very 

 probable that the corresponding littoral deposits of conglomerate 

 may lie somewhere beneath the later Carboniferous formations of the 

 midland valley. Along the northern margin of the midland valley 

 no evidence has yet been found of the Upper Silurian rocks as seen 

 at Leshmahagow, though some of the basal conglomerates may be 

 upon the same horizon, representing the littoral deposits to the 

 north of the Leshmahagow beds on the south. The occurrence of 

 Pachytheca in the sandstones at Murthly seems to indicate that the 

 old Upper Silurian flora had not yet passed away when these rocks 

 were deposited. The discrepancies between the fossils of Lanark- 

 shire and Forfarshire on the one hand, and between the fossils 

 of Forfarshire and Caithness on the other, are so great that we 

 are driven to the conclusion that the three must represent different 

 horizons of a gradually ascending and oveidapping series of beds, 

 each marked by a distinctive fauna and flora, and also by strong 

 lithological differences. The following section will better explain 

 our position. 



During Upper Silurian times, the mountain chain was depressed 

 as far as the point marked A, these deposits being now shown 

 passing up into the Old Red Sandstone. By the time the chain had 

 been depressed as far as B, the Silurian types had passed away, and 

 were now represented by those organisms found in Forfarshire. 

 Finally, the whole mountain mass was so far depressed as to 

 cover the- entire highland area as seen at C, when the Forfarshire 

 types had, in their turn, been superseded by those of Caithness. 



