Vol. 51.] RADIOLARIAN ROCKS IN THE LOWER CULM MEASURES. 631 



and minute crystal-needles of rutile and zircon. The siliceous 

 groundmass in polarized light, between crossed nicols, usually 

 exhibits the faint speckled aspect of cryptocrystalline silica similar 

 to that of a section of a flint from the Chalk ; sometimes it is almost 

 entirely dark, in other instances certain bands of the rock in which 

 the radiolaria are very crowded show the lively tints of chalcedony. 

 The radiolaria in the rock generally have been infilled with clear, 

 nearly transparent silica, free to a great extent from the rutile- 

 crystals and the dark substances disseminated in the groundmass, 

 and either cryptocrystalline or microcrystalline in character. 

 Within the radiolarian casts the silica has not unfrequently a radiate 

 fibrous arrangement, and in this case a black cross is shown in 

 polarized light. The more distinctly crystalline structure of the 

 radiolarian casts facilitates their recognition in the rocks with a 

 clear groundmass, where in ordinary light they are scarcely visible, 

 but between crossed nicols they appear as so many circles of 

 speckled or bright light on a nearly dark ground. 



These rocks, more particularly the harder and more cherty 

 varieties, very frequently contain minute casts of rhombohedral 

 crystals, ranging from '015 to '065 mm. in diameter, which are irre- 

 gularly scattered in the siliceous matrix. Probably they were origin- 

 ally of carbonate of lime or dolomite, which has now been dissolved 

 and removed. They not only occur in the groundmass of the rock, 

 but also within the casts of the radiolaria enclosed by the siliceous 

 infilling, and in some cases they seem to have become filled with 

 silica themselves. Dr. Rust * notes the occurrence of similar 

 rhombohedra of caleite within the casts of the radiolaria of the 

 Culm in the Harz, and there can hardly be any doubt that the 

 crystals 2 must have been formed where they, or the empty casts, 

 now occur. 



Microscopic cubes of pyrites are present in some of the rocks, 

 and, like the calcite-crystals or casts mentioned above, they are not 

 infrequently found within the radiolaria, enclosed by crypto- 

 crystalline silica either singly or in small aggregates (fig. 3). 

 There are, further, in some of the harder and more cherty beds, 

 some very minute rounded bodies, ranging from -006 to '013 mm. 

 in diameter, either detached singly or grouped together in 

 small masses ; these, in size and other characters, bear a 

 general resemblance to the small bodies in the pre-Cambrian 



1 ' Palfeontographica,' vol. xxxviii. (1892) p. 116. 



2 Similar rhombohedral crystals and casts are very generally found in 

 siliceous rocks of organic origin, and the carbonate of lime of which they 

 consist may well have been derived from the remains of calcareous organisms 

 associated in the same rock with the siliceous sponges, radiolaria, or diatoms, 

 but in much smaller proportions. The crystals are likewise found in some 

 siliceous rocks, such as the novaculites of Arkansas, in which no siliceous 

 organisms have as yet been recognized, and Mr. F. Rutley has lately contended 

 that the crystals are merely the residue of an original dolomite or dolomitic 

 limestone which has been replaced by silica (Quart. Journ. Greol. Soc. vol. 1. 

 1894, p. 382). A more probable explanation is that the novaculites were 

 originally organic — the same as the radiolarian beds of the Harz and Devon. 



2y 2 



