Lower Palaeozoic Rocks of the South of Scotland. 41 



nite zone of this Radiolarian cliert 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. Tliis 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 tlie aid of an ordinary hand-lens the fractured 

 surface of the chert is seen to be filled with countless numbers 



