Vol. 57.] SILURIAN [?] ROCKS IN FORFARSHIRTI, ETC. 335 



the rocks have undergone gives them the appearance of an older series ; 

 but notwithstanding this fact, there is strong presumptive evidence 

 that the cherts and green igneous rocks in the Soutliern Uplands and 

 along the Highland Border are of the same age (A.renig). 



Attention must now be directed to the question of the age of the 

 Margie Series. The green conglomerate, as alreadj' indicated (pp. 330, 

 331}, proves conclusively that these sediments are younger than the 

 Jasper and Green-Rock Series. On the other hand, it is beyond 

 doubt that the Old Eed Conglomerate has not shared in the powerful 

 movements that affected the Margie Grits. The relative age of the 

 two is, however, placed beyond dispute by the fact that, in the area 

 south-west of the North Esk, the Old Red Conglomerate passes 

 unconformably over the upturned edges of the Margie Grits. The 

 latter are, therefore, presumably of Silurian [?] age, though younger 

 than Arenig time. 



IV. The Nature of the Northern Boundary of the Silurian [?] 



Rocks. 



Having established the order of succession of these rocks, and 

 determined their age as far as possible, we will now consider their 

 relation to the Highland Series. 



Between the Highland rocks on the one hand, and the Jasper and 

 Green Rocks on the other, are several small lenticular patches of 

 the Margie Series which are shown on the Geological Survey Map 

 (Sheet 66) and lettered b. These are five in number, and occur at 

 the following localities :— (1) North Esk ; (2) Kirkton ; (3) Crichie 

 Burn ; (4) Clattering Bridge Quarry; and (5) Burnieshaig Quarry, 

 north of Hrumtochty Castle. The phenomena observed on the 

 • margins of these patches are so important as to justify me in 

 entering into the following details : — 



(1) The North Esk. — On referring to the description already 

 given (p. 331) of the northern outcrop of the Margie Series in 

 this section, it will be seen that there is a marked inversion of the 

 strata near their northern limit. On the northern face of the old 

 quarry the Margie Limestone, dark shale, and chocolate-coloured 

 shale are repeated in reverse order with a north-westerly dip (see 

 fig. 1, p. 330). It is clear that the beds north of the quarry are 

 arranged in descending order, although from the apparent dip we 

 might at first conclude that there is an ascending se(iuence. It is 

 further evident that there can be no natural sequence between the 

 chocolate-coloured shale at the northern limit of the Margie Series 

 and the Highland Slate a few yards to the north, for no representa- 

 tive of the Margie Grits occurs here. And as there is no ordinary 

 fault-rock present, it is most probable, in view of the great 

 deformation of the strata, that the junction is a thrust-plane. 

 But even supposing that this inversion could not be proved, the 

 lithological evidence would demonstrate the fact that there is no 

 true upward succession from the sheared Margie Shale, with its 



