OLD RED SANDSTONE OF WESTERN EUROPE. 391 
ing the base. It is on the coast from Brough to John o’ Groat’s that the most 
complete sections of the group occur. 
In comparing these sections, the first feature to arrest attention is the 
remarkable overlap of the beds upon the old crystalline platform to which 
reference has already been made on p. 373. The flagstones which, towards the 
east, retain the usual normal characters of fissile calcareous strata, pass into 
sandstones and conglomerates as they approach and rest upon the granite and 
gneiss (A in fig. 4). Hence, from Reay south-eastwards, these latter rocks are 
fringed with sandy and pebbly strata, not, however, as a continuous mantle 
sloping away from them, but in successive beds, abutting against the eastern 
edge of the crystalline era, and dipping towards the north, north-west, or 
north-east. Such a structure, as I have before remarked, might at first be 
attributed to the effect of a fault between the two masses of rock, but many 
natural sections may be examined where the pebbly strata rest directly upon 
and are formed out of the crystalline rocks beneath them. The subjomed map 
will explain this interesting and somewhat uncommon structure. 











































































































































































Fig 4.—Map of Part of the North-West of Caithness, showing the overlap of the Thurso Flagstone 
Group upon the Crystalline Rocks. A. Schists and Crystalline Rocks. B. Old Red Sandstone. 
The dotted lines mark the strike, and the arrows the dip of the strata. 
For the sake of clearness in description it will be most convenient to trace 
' first the section of the strata exposed to the west of Dunnet Bay, and then that 
on the east side between the two headlands of Dunnet and Duncansbay. 
Beginning on the shore at Castletown, we meet with thin-bedded and shaly 
flagstones dipping gently to the north-west. These strata have long been 
extensively quarried here for the well-known Caithness flags of commerce. 
They are also exposed further inland in the large quarries of Stonegun and 
