58 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 16, 



responds. Colonel Portlock draws attention to the strongly marked 

 line of demarcation between tlie fauna of the ancient rocks in this part 

 of Ireland and that of the formations which succeed them. " In 

 fact," he says, *' there is here no such intermediate formation, in a 

 fossil sense, as the Devonian system ; that is, there is no formation in 

 which fossils peculiar to itself are commingled with a large percentage 

 of those belonging to the Silurian on the one hand, and the carboni- 

 ferous on the other*." And this may with equal truth be asserted 

 of the south of Scotland, where the break both physical and paleeon- 

 tological, between the Silurian and the next higher formation, is re- 

 markably distinct, and indicates a long period during which no depo- 

 sits have been here formed. It was probably during this interval that 

 the rich ichthyohtic beds of Perthshire, Forfarshire and the north of 

 Scotland were accumulated. It is in the latter locahties therefore 

 that a transition downwards from the well-known carboniferous and 

 old red sandstone forms of life into those of the Silurian beds must 

 be sought, rather than on the southern side of the synchnal trough f. 

 Any estimate of the thickness of these Silurian deposits must evi- 

 dently be very imperfect, as the thick covering of detritus renders it 

 almost impossible to work out continuous sections. The difficulty is 

 increased by the principal rivers flowing generally along the strike of 

 the beds, so that those transverse gorges in which fall displays of the 

 stratification might be expected to occur are veiy rare. The follow- 

 ing calculations, therefore, are merely hypothetical, and intended 

 rather to stimulate than to satisfy inquiry. Assuming that the 

 Thornielee slates belong to a different part of the series from those of 

 the Grieston, we have in this part of Scotland three distinct bands 

 of fossiliferous rocks running from south-west to north-east in nearly 

 parallel lines. The most northerly is the Wrae limestone, which, in a 

 comitry where lime is of so much value, we may well believe would have 

 been known had it again cropped out to the south. It can be cer- 

 tainly traced for more than a mile, having been quarried on the south 

 side of the Tweed near Drumelzier Castle, where it is also accom- 

 panied by trap. A similar trap rock is seen twelve miles east in the 

 Eddleston river associated with an impure hmestone formerly quarried, 

 which probably forms the continuation of this bed. The distance of 

 these three bands from each other, measured on the map, at right 

 angles to their strike, is six miles from the first to the second, and 

 four miles from the second to the third, or ten miles in all. Allow- 

 ing for the inclination of the strata, supposed to dip at an average 

 angle of 50°, which is much below the reahty, the beds included in 

 these two zones will have a thickness of about 40,000 feet, or of 

 24,000 feet in the more northern one alone. As this is exclusive of 

 the whole mass of more recent beds on the south, the Silurian for- 

 mations of Scotland at least equal those of other countries in the 

 amount of accumulations, however inferior they may be in abundance 

 of organic remains. 



* Report on Lond. p. 233. 



t The "enormous aggregate thickness of the former deposits" was noticed twenty 

 years ago by Prof. Sedgwick and Sir R. I. Murchison. See Trans. Geol. Soc, 2nd 

 Series, vol. iii. p. 141. 



