944 DR ROBERT CAMPBELL ON 
doubtless under the thick mantle of drift which here completely obscures the solid 
geology. Reappearing again at the Knock Hill, they may be followed as occasional 
flows intercalated in the voleanic conglomerates along the slopes of the Haerscha Hill 
to Paldy Fair Den. Then, circling round the Elfhill anticline, they cross the Bervie 
Water between Dillavaird Ford and Tipperty. Between that stream and the Highland 
fault they are again concealed under the drift. ‘This group of lavas undoubtedly 
thickens towards the south and east. 
If we exclude a few intercalated flows of doleritic basalt which occur chiefly at or 
near the base of the series, the lavas of the Arbuthnott group form an assemblage of 
types altogether different from those found in any other part of the Lower Old Red 
succession in Kincardineshire. Detailed descriptions of these will be given in another 
paper, but meanwhile it may be noted that they are mainly hypersthene-bearing 
andesites and basalts. At one extreme we find normal hypersthene andesites without 
olivine; at the other, hypersthene basalts containing much olivine and very little 
hypersthene. Numerous transition types are characterised by varying proportions of 
the above two constituents. 
Like the similar types in the Ochils and Cheviots, these hypersthene-bearing basic 
lavas are remarkably rich in chalcedony, fine red veinlets of which are usually to be found 
ramifying through the rock in every direction, while the vesicular porticns of the flows 
yield beautiful examples of agates in great variety and abundance. It is on this horizon, 
at Usan on the Forfarshire side of the Montrose anticline, that the vesicular lavas occur 
which yielded many of the finest specimens in the Heddle collection. 
That the lavas of this group underwent contemporaneous erosion, although not on 
the same extensive scale as those of the Crawton and Dunnottar groups, is seen at more 
than one locality, but particularly well in a stream section about 200 yards below the 
bridge near Biddrie. There between two of the flows occurs a bed of volcanic 
conglomerate containing numerous boulders derived from the slaggy portions of the 
underlying lava. The general assemblage of the pebbles in this conglomerate, together 
-with a typical intercalation of tuffs, recalls, however, the conglomerates of the Clattering 
Bridge section, which are believed to represent the Arbuthnott group on the north side 
of the Howe of the Mearns. 
‘‘Sandstone-veinings” are of frequent occurrence in the slagey upper portions of 
the lavas, the material of the veins consisting usually of hardened, red or green, fine- 
grained micaceous sediment. The green veins in one of the flows near St Cyrus show 
curiously contorted bedding, and they exhibit also vesicular structure. The vesicles 
are filled with agate material similar to that found in the adjacent lava. 
In Paldy Fair Den I have mapped a flow which possesses somewhat unusual 
characters. It is, or at least it had been originally, a very vesicular vitrophyric type, 
with abundant phenocrysts of plagioclase and scattered phenocrysts of hypersthene and 
augite. But its most striking feature is the abundance of xenoliths of rounded 
boulders, mainly of hornblende andesites; in places the xenoliths are so numerous that 
