Lower Palceozoic Rocks of the South of Scotland. 41 
nite zone of this Radiolarian chert over a considerable area. 
The zone is bounded below by a thin band of black shale 
containing Glenkiln graptolites of Llandeilo facies, and above 
it there is another mass of black shale with Lower Hartfell 
fossils, having a Caradoc facies. The zone included between 
these two beds of graptolitic shale consists, from below up- 
wards, of nodular red and green cherts and red and green 
mudstones, followed by massive grey mudstones and cherts, 
mudstones and shales, succeeded above by black flints and 
shale, with a few Glenkiln graptolites. This Radiolarian 
zone of Mr. Peach thus corresponds with the Lower and part 
of the Middle Division of the Moffat Terrane of Prof. Lap- 
worth (Geol. Mag. dec. iii. vol. vi. (1889) p. 66). Hitherto 
in this series of rocks the graptolitic zones have been chiefly 
studied and the intermediate beds of chert, regarded as unfos- 
siliferous, have been neglected ; but it is now certain that 
these latter are of organic origin equally as much as the 
former. 
The Radiolarian chert is a very hard compact rock, with 
the usual hackly fracture ; when unweathered it is for the 
most part of a steely-blue tint, but sometimes of a dull to a 
bright red ; less frequently it has a greenish tint, and some 
pieces are even of a bright green. The rock is traversed in 
all directions by microscopic cracks and fissures, these latter 
now filled with crystalline quartz, and not unfrequently it is 
stained in irregular patches by a dark brown or blackish sub- 
stance, which often follows the course of the microscopic 
cracks, so that they appear in sections like an intricate web of 
dark threads crossing a clear field. The mudstones accom- 
panying the chert are greenish or reddish in tint and very 
fine-grained ; in some cases they become siliceous and pass 
gradually into chert ; in these transition-beds casts of Radio- 
laria are present in the rock. 
In thin sections under the microscope the unstained portion 
of the chert is nearly transparent ; it has a faint cloudy appear- 
ance, due to the presence of extremely minute irregularly- 
shaped mineral particles and small crystalline rods ranging 
from , 002 to *06 millim. in length, with which it is filled. 
The nature of these minute particles cannot well be ascer- 
tained ; but Mr. J. J. H. Teall, F.R.S., who has examined 
the sections, thinks that some may be flakes of mica, whilst 
the rods are suggestive of rutile. In polarized light, between 
crossed Nicols, this chert has a mottled appearance, more like 
that of flint than of ordinary chert. 
Even with the aid of an ordinary hand-lens the fractured 
surface of the chert is seen to be filled with countless numbers 
