160 
SILURIA. 
[Chap. VIII. 
Fossils (25). 
» Orthoceras Maclareni. Found in 
II the Upper Silurian strata of the 
II Pentland Hills. 
Lanarkshire. — Unacquainted with any of the intermediate strata which 
may be found when these Upper Silurian rocks shall be traced from Edin- 
burghshire on the E.N.E. to Lanarkshire on the W.S.W., I satisfied myself 
that in the latter county, to the west of Lesmahago, there is an ascending 
passage upwards from clay-slates with calcareous nodules and a rare Ortho- 
ceras, into black schists with large Crustaceans, which manifestly stand 
in the place of the uppermost course of the Ludlow rocks of Shropshire. 
This important fact was discovered by Mr. Robert Slimon of Lesmahago. 
The relations of these dark-grey schists to the inferior rocks, as well as 
to the overlying Old Red Sandstone, are seen on the banks of the Nethan 
River and other tributaries of the Clyde, particularly the Logan Water. 
The accompanying diagram (p. 161) will fully explain the relations of 
these Upper Silurian rocks to the overlying masses of Old Red Sandstone 
and Carboniferous rocks. It was in the beds a that Mr. Slimon detected 
the remarkable Crustaceans which I then acquired from him, and which, 
being deposited in the Museum of Practical Geology, were first partially 
described by Professor Huxley and Mr. Salter in the Journal of the Geo- 
logical Society, as a sequel to the memoir wherein the geological relations 
of the tract were explained by myself*. 
Rising out from beneath the Lower Old Red, upon the Logan Water 
banks, these dark -grey beds occupy precisely the same horizon as that 
uppermost zone of the Silurian rocks of Shropshire and Herefordshire 
which includes the Bone-bed and the Downton Castle building- stone, and 
to which, where it graduates up into the Old Red, the names of ' tile- 
stones' and ' passage-beds' have been given, as before explained. We have 
even the characteristic shells of the zone — Platychisma helicites and Lin- 
gula cornea. 
The large Crustaceans which chiefly characterize the dark clay- slate, b, 
belong to those Pterygoti which occur, as we have seen, at intervals 
throughout the Upper Silurian rocks and are strikingly prevalent in their 
uppermost beds. In the woodcut at p. 162 are represented the principal 
species discovered in Lanarkshire. They appear to be specifically distinct 
* Professor Eamsay and myself visited the loca- vol. xii. p. 17). Since that memoir was printed, 
lities near Lesmahago accompanied by Mr. Robert Mr. G-eikie has published a more detailed account 
Slimon; and afterwards preparing the accom- of the structure of the Upper Silurian rocks and 
panying section, I published it in a short me- Lower Old Eed Sandstone of the Lesmahago tract 
moir descriptive of the general relations of the (Quart. Journ. Geol. Soc. vol. xvi. p. 312). 
deposits in this tract (Quart. Journ. Geol. Soc. 
