308 C. Lapivorth — On Scottish Monograptidce. 



downwards, may be inferred from the fact that this latter position, 

 where the base would be upwards, would raise the centre of gravity 

 nearer to the surface of the water, and thus bring about a condition 

 of equilibrium not so stable as if the apex had been upwards. 



These considerations, it must be borne in mind, are purely theo- 

 retical, and are o\Ay used as a means of giving a clearer idea of some 

 of the conditions under which ice may be found floating, and especi- 

 ally such ice as has suffered disintegration in its wanderings. 

 {To be continued in our next Number.) 



V. — On Scottish Monograpttd^. 



By Chas. Lapwobth, F.G.S. 



(PLATES IX. AND X.) 



§ I. — Introduction. 



1. Succession of Silurian Bocks in the South of Scotland. — The 

 oldest fossiliferous strata yet detected in the Silurian district of the 

 South of Scotland are the dark Graptolitic shales forming the 

 well-known Moffat Series. The total thickness of this group is 

 under 600 feet, but it is nevertheless capable of separation into 

 three very distinct palseontological divisions, each characterized by a 

 large proportion of peculiar fossils. The first of these divisions is 

 of Llandeilo age, the second represents a portion of the Bala forma- 

 tion of North Wales, and the third, or Birkhill division, appertain^, 

 at least in part, to the epoch of the Lower Llandovery. 



But the formation of the highest importance geographically is a 

 group of comparativel}' unfossiliferous shales and greywackes of 

 enormous thickness ; which — reposing on the highest division of the 

 Moffat Series in some localities, and on the middle division in others 

 — stretches in an almost unbroken sheet from St. Abb's Head to the 

 Mull of Galloway, and occupies at least nine-tenths of the whole 

 Silurian area. Its rocks are thrown into innumerable folds and con- 

 tortions, and the inferior strata are exposed only in long narrow bands 

 in the lines of the deeper anticlinals. This great group appertains 

 wholly to the Middle Silurian epoch. To its lower division belong 

 the Girvan rocks and the so-called Gala Group of the eastern and 

 central districts. Its highest beds are the Hawick Eocks, formerly 

 believed to be the most ancient strata in the South of Scotland. 



To the south of the Hawick Eocks, and reposing conformably 

 upon them, lie the more fossiliferous Balmae or Eiccarton beds, 

 which are of Wenlock age, and floor a long and narrow strip of 

 country stretching from Kirkcudbright to the central portions of 

 the Cheviots. Strata of Ludlow age occur only in the northern 

 areas, where they emerge from below the Upper Palaeozoic rocks of 

 Lesmahagow and the Pentland Hills. 



2. Distribution of FJiahdophora in these deposits. — Graptolites are 

 met with upon several horizons in this succession. They are 

 especially numerous in the Moffat Series, in all the divisions of 

 which they occur in countless multitudes. In the Gala and Girvan 

 beds they are found but sparingly, and in a few thin carbonaceous 

 bands only. The Hawick rocks, though frequently swarming with 

 the enigmatical Protovirgularia — like their equivalents in Wales and 



