ON THE PLANT-BEARING CHERTS AT RHYNIE, ABERDEENSHIRE. 213 



Flag group, and evidently underlie the plant-bearing cherts laid bare 

 in the trenches in the field to the north-east. 



Further up stream, at a point about 500 feet from the datum line, 

 calcareous shales about 10 feet in thickness appear on the right bank, 

 followed by flaky sandstones. The shales and sandstones are vertical, 

 and have a nortli-east and south-west strike — features that suggest 

 proximity to a fault. The strike of these beds is parallel to the trend 

 of the boundary fault on the western margin of the Ehynie outlier of 

 Old Eed Sandstone. Immediately to the west, a band of hornblendic 

 andesite crosses the stream. Its stratigraphical horizon is not clear, 

 but it is referred provisionally to the Old Red Sandstone of the Rhynie 

 outlier. 



iii. Roadside Section, Craigbeg. 



With the sanction of the road authorities, the rocks were laid bai'e 

 by the side of the road ascending Craigbeg between Rhynie and the 

 farm of Newseat (fig. 2). On its north-east side the rocks form a 

 steep bank, covered in part by soil and vegetation. The vegetation 

 was removed, and a continuous rock section, 110 feet in length, was 

 exposed. In a south-east direction, where the gradient is not so steep, 

 this rock section ended in superficial materials. Trenches were dug 

 to find the solid rock between this locality and trench No. 3 (the 

 nearest point to the south-east at which rock was found), but without 

 success owing to the covering of drift. 



The interest of the roadside section centres in the following points : 

 (l)The position of the fault between thediorite and the Craigbeg Series 

 (the ' Older Series ' of Dr. Mackie), (2) the junction of the rhyolite 

 and the ' Lower Grits '), (3) the probable position of the fault between 

 the Old Red Sandstone of the Ehynie outlier and the ' Older Series,' 

 and (4) the exposure of the chert band and other members of the Dryden 

 Flag group. 



Beginning at the diorite at the north-west end of the road section 

 and descending towards Rhynie, we pass from lower to higher beds. 

 Dips are, however, only plainly seen in the beds that overlie the 

 rhyolite, their inclination increasing from 25° to about 40° where the 

 section ends. 



(1) The fault between the diorite and the Craigbeg Series was located 

 at 1,045 feet from the datum line of the Cross Ditch. Its hade is about 

 33° to the east. Its position is defined by a band of dark-purplish 

 clay which was excavated at the top of the bank and at the road-level. 

 Here the ' Slit Rock ' of Dr. Mackie 's succession lies against the fault 

 plane, the basal chert of the ' Older Series ' being cut out by the fault. 

 But it appears in place at the north-east corner of the old diorite quarry 

 about 20 yai'ds to the north of the point indicated. 



(2) South-east from the fault, the ' Lower Grits ' of Dr. Mackie 's 

 succession are exposed in the bank above the road, and their junction 

 with the overlying rhyolite was laid bare for a distance of three or four 

 feet, about 300 yards from the datum line. The junction line is more 

 or less vertical, but follows an irregular zig-zag course. The two rocks 

 are welded together, and the ' Lower Grit ' is bleached to a depth of 

 about an inch at the point of contact. Dr. Flett and Dr. Campbell 



