Vol. 51.] RADIOLARIAN ROCKS IN THE LOWER CULM MEASERES. 633 



singular absence of other microscopic organisms in these Culm 

 siliceous rocks, and the only forms met with are a few minute 

 dentated plates of a dark or brownish tinge which have a general 

 resemblance to the radute of gasteropoda. Bodies of a similar 

 character are occasionally present also in the Ordovician radiolarian 

 rocks of the South of Scotland. 1 



In the greater number of the microscopic sections of the Eadiolarian 

 rocks which we have examined, detrital materials— beyond perhaps 

 some minute fiakelets of mica — are not shown ; but in some beds, 

 more particularly in the hard, dark, platy varieties, there is a certain 

 admixture of minute chips of quartz and flakes of mica which appear 

 to be of detrital origin. The particles are very small, ranging from 

 •03 to "065 mm. in diameter, and thus not larger than the detrital 

 grains which occur in typical recent radiolarian ooze or in the radio- 

 larian rocks of Barbados. Badiolaria are present in the same rock 

 with these detrital grains, but they are less numerous than in the 

 rocks in which they are not found. 



The very generally siliceous nature of the Radiolarian rocks has 

 been already mentioned : exceptions to this occur in the exposures 

 near Lawell House, Chudleigh, where there are some beds of dark 

 , limestone alternating with siliceous beds; the limestones contain 

 casts of radiolaria and sponge-spicules which in some instances are* 

 replaced by calcite, in others the radiolaria are infilled either with 

 microcrystalline silica or partly with this mineral and partly with 

 calcite. The limestone-matrix is finely granular, and some indeter- 

 minable fragments of calcareous organisms are present in it. 



We have also examined microscopic sections of the dark limestones 

 which, both in the Launceston district and near Barnstaple, appear 

 below the Eadiolarian rocks. The limestone of the Wooladon quarry 

 near Launceston is finely granular, with some traces of calcareous 

 organisms and a few rounded bodies, now infilled with calcite, which 

 may possibly be casts of radiolaria. The dark limestone of the 

 Tinhay quarry, a little farther east, appears to be almost entirely 

 crystalline, and no organisms can be seen in it. A thin section of 

 the black limestone of Swimbridge near Barnstaple was fine-grained 

 and filled with fragments of entomostracan shells and crinoids, also 

 a few rounded bodies of uncertain character. The limestone at 

 Venn, near Barnstaple, is very similar to that at Swimbridge. 



The bluish-grey limestones of Canonsleigh, near Burlescombe 

 Station, are very crystalline ; they contain, however, foraminifera of 

 the genus Endothyra. In sections of the hard, grey, cherty layers 

 associated with these limestones the matrix is of cryptocrystalline 

 silica, in which there are numerous rhombohedra of calcite and casts 

 of radiolaria, precisely similar to those in the normal Eadiolarian 

 rocks. 



VII. Description op the Badiolaria. 



Although there are many varieties of the Culm radiolarian rocks, 

 and these organisms are abundant in all, we have not succeeded in 



1 Ann. & Mag. Nat. Hist. ser. 6, vol. vi. (1890) p. 45. 



