wilmams.] THE TILESTONE OF THE SILURIAN SYSTEM. 83 
selves; but, on passing below, -the typical Upper Ludlow weathered 
gray. This seems to have been the prime reason in the mind of Mur- 
chison for classing the Tilestone with the Old Red. When, however, 
the first edition of Siluria was written, the fact that the species of 
this fauna were marine and not fresh-water types, and that some of 
them were identical with species in the formation below (whi» h he 
called Silurian), led Murchison to call this Tilestone the top member of 
the Silurian instead of the bottom of the Old Red. In Siluria appears 
a concise description of the transition from the Ludlow rocks into the 
Old Red sandstone, as seen in Carmarthenshire and Pembrokeshire: 
" In all these places strata of dull greenish-gray argillaceous sandstone, 
minutely micaceous, differing chiefly from the t}^pe of the Ludlow 
of Shropshire in being harder and thicker bedded, and which repose 
on rocks with Upper Silurian fossils, plunge under red and green strata 
(the red rabof Pembroke), or bottom beds of the Old Red sandstone" 
(p. 141). 
The 'typical localities from which the Tilestone fossils came are 
Felindre, on the Teme, and Horeb Chapel, in the valley of Cwm Dwr, 
between Trecastle and Llandovery, near Cwm Dwr. In The Silurian 
System the passage beds near Felindre are described as ; ' hard, green- 
ish, and reddish, highly micaceous sandstone, which contain the Lep- 
tcena lata and the Terebratula nucula of the Ludlow rock, together 
with casts of several shells identical with those found in the tilestones 
of the Cwm Dwr, Carmarthenshire, and which have never been found 
in the Silurian system below its junction with the Old Red sandstone" 
(p. 191). 
The reason for placing the base of the Old Red sandstone above these 
tilestones is stated in the following passage in Siluria: "Even then, 
however, the fossils which were figured as characteristic of such tile- 
stones exhibited little else, as I showed, than species common to the 
Ludlow rock itself. This zoological fact, and subsequent researches 
in other parts of England, above all those of Professor Sedgwick in 
Westmoreland, where the Upper Ludlow strata are much developed, 
have for eleven years led me to classify these tilestones with the 
Silurian rocks, of which they form the natural summit. For, even in 
their range from Shropshire through Hereford and Radnorshire, into 
Brecon and Carmarthenshire, whether they are of red or yellow colors, 
they are charged with Orthoceras bullatwn, Chonetes (Leptama) lata, 
Spwifer elevatus, Orthis lunata, Bhynchonella nucula, Cucullella ovata, 
Better ophon trilobatus, B. expansus, Trochus helicites, Holopella (Tur- 
ritetta) obsoleta, and the minute bivalved crustacean, Beyrichia tuh, ,■- 
Gulata. All of these are the most common fossils of the Upper Ludlow 
rock; although a few of them descend as low as the Caradoc sand- 
stone" (p. 139). 
