Vol.51.] ON THE OCCURRENCE OF RADIOLARIA IN CHALK. 603 



contain 12 to 13 per cent, of colloid silica, but this proportion is 

 exceptional, and recognizable radiolarian forms are, as a rule, widely 

 separated. 



In the present communication we propose to describe only those 

 forms which we have found in the nodules of the Melbourn Rock, 

 leaving further notice of their occurrence in the Lower Chalk for a 

 future occasion, by which time we may perhaps have discovered 

 some clue to the relation between the occurrence of radiolaria and 

 the formation of the nodules in which they occur. 



III. The Instability of the Radiolarian Skeleton. 



Slices cut from some of the consolidated deep-sea deposits of 

 Barbados give valuable indications of the manner in which radio- 

 larians are gradually destroyed in the process of fossilization. Those 

 in the more purely siliceous earths do not seem to undergo much 

 change, but those contained in material which is partly siliceous 

 and partly calcareous are more rapidly altered. If a thin section 

 of a calcareo-siliceous rock be viewed by transmitted light, the radio- 

 larian is usually seen in annular outline marked by clear crystalline 

 material (which occupies the position of the original siliceous skele- 

 ton), while the interior is filled with a denser, amorphous, calcareous 

 matrix like that of the surrounding rock. The continuity of the 

 clear outline is broken by the pores of the test which, as well as 

 the interior, have become filled with the denser calcareous material. 

 Not infrequently the infilling matter is slightly more crystalline 

 than the matrix of the rock, and, the light passing more readily 

 through it, the outline of the radiolarian acquires greater definition.. 



But in some cases the interior of the test is filled entirely with 

 clear crystalline material, and it then stands out in strong relief to 

 the darker grey shade of the matrix. Between these two extremes 

 there is every gradation, the lattice-work of the test being some- 

 times partly visible and sometimes partly destroyed, in others so 

 completely obliterated that nothing but the outline of the test re- 

 mains for recognition. 



The crystalline material infilling the tests of radiolaria in the 

 rocks of Barbados may be either calcite or silica in the colloid state,, 

 according to the nature of the rock : calcite where calcite predomi- 

 nates, silica where the rock contains the greater proportion of 

 radiolarians ; but often in these mixed rocks the polariscope proves 

 both calcite and silica in the same test. It is evident that the 

 siliceous skeleton of the radiolarian enters into solution and is re- 

 deposited in the matrix of the rock, binding its ingredients into a 

 compact calcareo-siliceous mass ; this material fills the chambers of 

 the foraminiferal shells and the tests of the radiolaria. 



We also observe that the preservation of a radiolarian, even in 

 the form of a cast, depends greatly on the shape and structure of 

 the test. From the examination of a number of slides of the Bar- 

 badian rocks we find that Sphaeroid, Discoid, and Prunoid tests are 

 the most prominent, the enclosed cell-like character of these shapes 

 seeming to favour the accumulation of glassy material within them. 



