ON THE GEOLOGY OF BAKBADOS. 175 



except a vague outline is obliterated. It is certain also that in 

 these mixed deposits the radiolarian skeleton either enters into 

 solution or into a chemical combination with the surrounding lime, 

 and that the silica, in a form not visibly acted on by hydrochloric 

 acid, is redeposited in the interior both of radiolarians and forami- 

 nifera. Thus in the calcareo-siliceous earths it frequently (not 

 always) happens that while the radiolarian is unmistakably out- 

 lined in the thin section, the siliceous skeleton has disappeared, its 

 place and the interior of the original test being filled with a clear 

 crystalline material. 



Foraminifera, on the other hand, are nearly always well pre- 

 served ; their tests lose the hyaline appearance of living or recently 

 dead individuals, and become impregnated with crystalline carbonate 

 of lime, but retain the details of their minute structure. It is 

 worth noting that both their outline and structure are clearer in 

 mixed than in purely calcareous deposits. 



(2) Silicified Earths. A siliceous limestone from the base of 

 Clelancl Hill. — In hand-specimens this rock looks like a dense hard 

 white limestone. Examined under the microscope by means of 

 thin sections it is seen to be one of the siliceo-calcareous deposits, 

 in which many entire radiolarians can be recognized, and in which 

 foraminifera, chiefly Glohigerince^ are not uncommon. 



There is also much fine calcareous matter, and to the eye the 

 character of the rock does not seem to differ materially from the 

 softer siliceo-calcareous deposits. 



By treating a very thin slice of the rock in a solution of hydro- 

 chloric acid, though there was a strong reaction, the slice did not 

 break down. A thin section of the remainder showed that the acid 

 had removed a large part of the calcareous matter, and that the 

 rock was permeated with a glassy material not acted on by the 

 acid, and neutral to polarized light. The tests of the foraminifera 

 are partially replaced by this material, which has also penetrated 

 the foramina and the interior spaces of the cells. 



Another part of the slice, previously treated with acid, easily dis- 

 integrated in a heated solution of caustic potash. 



From its optical and chemical properties it would appear that the 

 cementing material of the rock is of the nature of colloid silica. 



Siliceous Concretions. — In the siliceous and calcareo-siliceous 

 rocks there occur nodules, some of which in their general appearance 

 strikingly remind one of the nodules of immature chert found in 

 the Chalk Marl of Wiltshire and in the Malmstone of Hampshire. 



The nodules which bear the strongest resemblance to flint in 

 their appearance and fracture are from the Eadiolarian Marl of 

 Springfield, a purely siliceous deposit. 



These, when examined in section under the microscope, are seen to 

 have been formed by the cementation of radiolarians and other 

 siliceous organisms by glassy silica, the whole being neutral to 

 polarized light. There is no reaction when a thin slice is treated 

 with hydrochloric acid, but the mass disintegrates in a heated 

 solution of caustic potash. 



Bluish chcrty nodules from the calcareo-siliceous deposit of Castle 



