56 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 16, 



In Thornielee slate quarry in Selkirkshire, on the bank of the Tweed, 

 about six miles below Grieston, a few specimens of two of the same 

 species of graptolites were found. The beds here dip at 82° to S. 50° 

 E., or in the opposite direction to those at the former place, and also 

 lie considerably south of their strike, so that they may thus form 

 merely the other side of an anticlinal axis. They have, however, a 

 different mineralogical aspect, being often of a broTvaiish red colour, 

 so that I am more inclined to consider them distinct. Besides the 

 G. convolutus and the G. ludensis, there is probably a foliaceous spe- 

 cies of this genus, which is not seen in the Grieston slates. In the 

 latter also annelid impressions are rare, whereas in Thornielee quarry 

 they are very abundant, thus strengthening the \iew that these two 

 localities belong to distinct parts of the series. Some of these worm- 

 like impressions much resemble the species figured in the * Silurian 

 System ' of Sir Roderick Murchison, and may even be identical ; but it 

 is difficult to obtain certainty in regard to forms presenting so few 

 well-defined characters, that even their animal nature may be doubted. 

 The regularity of the folds, with the apparent impression of feet or 

 setss on the margin, seem to show that they are organic, and rather 

 an impression of the body of the animal itself than a m^ere trail left 

 in the soft mud*. Similar fossils occur in the Tyrone beds, which, as 

 Colonel Portlock states f, " exhibit on their surfaces those markings 

 like the stems of algee and the tortuous labyrinths of annelids, so 

 common to indurated muddy or sandy strata." They have also been 

 observed in the older palaeozoic rocks of North America, and several, 

 not unlike those from Scotland, are figured by Mr. Emmons from his 

 so-called Taconic group. Thus far they confirm the view now given 

 of the age and connexion of these rocks ; though as similar rude me- 

 morials of extinct life occur in formations of every age, they perhaps 

 rather indicate similarity of conditions during deposition than identity 

 ofage:!:. 



From the whole facts noticed in this and a former paper, there can 

 be little doubt that the slates and greywackes of this part of Scotland 

 belong to the Lower Silurian period, and are probably the equivalents 

 of the Llandeilo flags of Wales. Judging from the specimens of the 

 older Welsh rocks contained in the Museum of the Society, these 



several veins that intersect the strata at a considerable angle. Veins of this kind 

 may be seen in Priesthope, though most of them in that locality conform to the 

 bedding. A vein near the source of the Leithen with a direction to S. 50*^ E. by 

 compass, whilst the strike of the strata is nearly from E. to W,, also confirms this 

 view. In a mass of vertical, or nearly vertical strata, the line of least resistance 

 to the escape of an igneous rock would of course be between the beds. 



* Some of these impressions penetrate a considerable thickness of the slate, 

 even as much as one-fourth of an inch. This shows that the animal has rather 

 lived in the mud than moved through it. The peculiar arrangement in tne mass 

 below might arise from the worm gradually raising.its body towards the surface 

 by a kind of undulating motion as the soft mud accumulated and deprived it of 

 access to the water. 



t Geol. Report on Londonderry, p. 230. 



X Similar forms are common in rocks of the coal formation and the lias, and spe- 

 cimens from both are preserved in the Museum of the Society. Sir Charles Lyell 

 recently presented others from modern mud deposits in the Bay of Fundy. See 

 Quart. Geol. Journ, vol. v. p. 344. 



