ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxix 



iu Dumfries-shire and to put it in relation with that portion of the 

 axis which had been determined by Professor Nicol in Roxburgh- 

 shire ; and his inference from the comparison of these two portions of 

 the axis is, that it has an E.N.E. and W.S.W. direction in the South 

 of Scotland. The lithological nature of the rocks which actually 

 constitute the axis, he represents as " purple grits, which have great 

 resemblance to some of the bottom-rocks of the Longmynd. Rest- 

 ing uniformly on this axis, on each side, are thin-bedded sandstones 

 alternating with grey and purplish-red shales, the purplish-red shales 

 abounding more in the lower portion of the series than in the upper, 

 where anthracitic and graptolitic shales occur ; the purplish-red 

 shales, therefore, marking a low zone in the Silurians of the South 

 of Scotland. These three rocks extend over a considerable lateral 

 space : Professor Harkness considers that their actual thickness is 

 small, and that the multiplied stratification is due to flexure from 

 lateral pressure, and the consequent frequent repetition of the same 

 beds. He also adduces a decided slaty cleavage in the argillaceous 

 strata as a further proof of pressure, although the absence of such 

 cleavage in the gritty beds had been previously cited as an argument 

 against the pressure-theory of cleavage, that absence being justly 

 explained on the principle that arenaceous beds are less susceptible 

 of cleavage than argillaceous, the reason of which I have already 

 endeavoured to point out. 



On the south side of the axis in Roxburghshire occur ripple- 

 marked beds, which also exhibit what Professor Harkness con- 

 jectures must have been Sun-cracks from desiccation, and other ap- 

 pearances of a shore periodically laid bare, as also tracks of an 

 animal in progression, which Mr. Salter has named Protichnites sco- 

 ticus, considering it a Crustacean, with feet analogous to, but not 

 identical with, that described and named by Professor Owen from 

 the tracks in the Potsdam Sandstone. In the purplish-red shales, so 

 low down in the series, the Protovirgularia and some obscure Grap- 

 tolites occur ; and at Dalton in Dumfries-shire Graptolites have also 

 been found ; but it is remarkable that the more decided graptolitic 

 beds associated with anthracitic beds, which in the section are re- 

 presented as normally existing at a higher level on both sides of the 

 axis, have not been found on the south of the axis in Dum- 

 fries-shire. The existence of the Protovirgularia in the purplish-red 

 shales at the bottom of the series in Dumfries-shire, as well as in the 

 gritstone beds of Peebles-shire and the Barlae Flags of Kirkcudbright- 

 shire, and its absence in the anthracitic and graptolitic schists, have 

 induced the Professor to alter his former opinion, and to allot the 

 former to a very low zone in the series, corresponding to a similar 

 zone in Sweden and Norway, considering the graptolitic and anthra- 

 citic beds as belonging to a higher zone in the Lower Silurians. Of 

 the whole, however, he appears to consider the purplish-red shales 

 as the lowest in order, and therefore as indicating some of the earliest 

 traces of animal life hitherto discovered, and that under conditions 

 which prove, in the periodical desiccation of the shore, and in the mark- 

 ings of Crustaceans upon it, the close approximation to dry land. 



