BUFFALO SOCIETY OF NATURAL SCIENCES 1 25 



cance of this succession will be spoken of presently. Continuing at right 

 angles to the strike, there is found about 2 miles northwest of Hart- 

 fell on the Cow Linn, the last outcrop of the Monograptus gregarius 

 zone. Only 3! miles northwest of this locality, near the junction 

 of the Fruid water with the River Tweed the gregarius zone is no 

 longer to be found, the highest of the Birkhill beds being the Diplo- 

 graptus vesiculosus zone (a2) which is the second in the Lower Birkhill 

 series. This zone is immediately followed by the Tarannon grits. 

 As the last of the Llandovery outcrops are traced towards the north, 

 fossils become very rare indeed and, although towards the boundary 

 line of the northern and central belts no specimens of D. vesiculosus 

 or of D. acuminatus have been found, a few other graptolites which 

 along the valley of the Tweed are associated with these zonal fossils, 

 have been encountered. It is thus seen that within the remarkably 

 short distance of 9 miles, 6 as traced from the Dobb's Linn anticline to 

 the Llandovery-Tarannon border, the whole of the Upper and nearly 

 all of the Lower Birkhill shales have disappeared, the fossils becom- 

 ing rare even in the shale members which are found, and, most sig- 

 nificant of all, the Tarannon grits or conglomerates everywhere fol- 

 low upon whatever member of the Birkhill group forms the top of 

 the section. 



Such a stratigraphic relation might be interpreted in one of two 

 ways. On the one hand it might be supposed that the Llandovery 

 sea retreated to the southwest and that dry land conditions accom- 

 panied by subaerial denudation obtained in the areas laid bare. This 

 would imply that more of the Birkhill shales had been deposited to 

 the northwest of Moffatdale than are now seen in the sections and 

 that the present exposures represent merely the parts which have 

 not been touched by erosion. The Tarannon would then represent 

 the river deposits spread out upon the eroded remnants of the Llando- 

 very. That such is in all probability not the case is indicated by the 

 statement made by Peach and Home that in the Hartf ell section (in 

 the Frizzle Burn) "the black shales and mudstones of the Monograp- 

 tus gregarius zone pass conformably upwards into the massive grits 

 of Tarannon age without any representative of the Upper Birkhill 

 Shales" (P. and H. 215, 133). The significant word is conformable. 

 If the contact is conformable there was no erosion, and therefore it is 

 not likely that two miles distant there was any considerable erosion. 

 Thus another interpretation is called for. The facts, and they have 



6 The distance would, of course, be much greater were the folds eliminated. 



