Vol. 51.] RADIOLARIAN KOCKS IN THE LOWER CULM MEASURES. 629 



"V. Chemical Composition oe the Badiolarian Rocks. 



We are indebted to Mr. J. Hort Player, F.C.S., for making two 

 analyses of the rocks, which are given below. 



I. II. 



Hard, dark chert Soft, grey shale from 

 from, Ramshom Barracadoes Quarry, 



Down. near Launceston. 



Silica 87-3 67-8 



Alumina 57 198 



Ferric oxide 1*3 14 



Ferrous oxide 14 4 



Lime '3 — 



Magnesia 12 12 



Potash -7 4-8 



Loss at 100° — -9 



Loss by ignition 1'9 36 



Totals 99-8 99-9 



Both these analyses are taken from specimens which, under the 

 microscope, can be seen to contain numerous radiolaria ; these were, 

 however, more abundant in No. II. The Eamshorn Down specimen 

 is a typical example of the hard, dark, brittle chert, and, as the 

 analysis shows, the proportion of silica is large. No. II. specimen, 

 on the other hand, is a typical example of the soft grey shales 

 which come in "between the harder beds. In this the silica is in 

 less proportion, while there are larger amounts of alumina and 

 potash, the latter originating, as suggested by Mr. Teall, from mica 

 present in the rock. The analysis of a siliceous slate (Wetz- 

 schiefer) from the Culm formation at Teufelsberg, near Lautenthal, 

 in the Harz, which contained very numerous radiolaria, is given by 

 Dr. Bust l as silica 69*9, alumina 17'6, soda 3*5. In this rock the 

 proportions of silica and alumina are nearly the same as in the 

 Barracadoes shale, but instead of potash it contains soda. 



A marked feature of the Radiolarian rocks is the very general 

 absence of carbonate of lime ; even in beds which contain casts of 

 brachiopods, crinoids, and other calcareous organisms, all traces of 

 lime seem now to have disappeared. The only exceptions met with 

 are in some of the rocks at Bamshorn Down and the Chudleigh 

 district, in which a small quantity of lime is present. 



VI. Microscopic Characters of the Badiolarian Bocks. 



For the study of the rock-structure and the radiolaria we have 

 had about 300 thin sections prepared from the typical rocks in the 

 different localities mentioned above. These show under the micro- 

 scope a general resemblance in consisting of a siliceous groundmass, 

 in some cases clear and transparent, in others dark and turbid from 

 the presence of fine particles of carbonaceous or ferrous materials 



1 ' Palseontographica,' vol. xxxviii. (1892) p. 111. 

 Q. J. G. S. No. 204. 2 t 



