Structure of Hearthstone from Betchworth, Surrey. 19 



removed to some distance from mouths of rivers bearing abundant 

 fine silt into the ocean. 



In the hearthstone, glauconite occurs in the form of short 

 cyHadrical rods, sometimes branched at one end, which have 

 evidently been formed in the cavities of sponge spicules ; and 

 also in more or less rounded grains, in some cases exhibiting 

 plainly the contour of foraminifera, in the cavities of which they 

 had been formed. 



The fourth portion consists of the smaller forms of foramini- 

 fera, such as globigerina and textularia, and of numerous disc- 

 like bodies or spherules, which are undissolved by acid, and are 

 stated by Dr. Hinde to be derived from the solution of the silica 

 of sponge spicules and of other silica in a hydrous state, and its 

 subsequent deposition in this form. Many of these spherules 

 exhibit a radiating structure, which is evidently due to a form 

 of crystallization. 



The fifth and finest portion consists principally of coccoliths 

 interspersed with rhabdoliths. These bodies are extremely 

 minute, and play a most important part in all deep-sea deposits, 

 with the exception of polar and subpolar regions. Coccoliths 

 are oval calcareous disks, having a thick strongly refracting rim 

 and centre, and are the disintegrated remains of a spherical 

 body known as a coccosphere. Rhabdoliths consist of minute 

 calcareous rods, having a disk at one end, and are the disinte- 

 grated remains of a body called a rhabdosphere. 



There has been considerable doubt as to the nature of cocco- 

 spheres and rhabdospheres, but they are now regarded as pelagic 

 algae. Rhabdospheres are developed in equatorial and tropical 

 regions, and are rarely met with where the temperature of the sur- 

 face-water falls below 65°. Coccospheres, while abundant in the 

 tropics, are found further north and south than rhabdospheres, 

 and are present where the temperature is as low as 45°. They 

 attain their greatest development in temperate regions, but are 

 absent in water affected by rivers. 



There is something very remarkable in the fact that such 

 minute and fragile bodies as coccoliths and rhabdoliths should 

 be able to withstand the action of time, when siliceous bodies 

 such as radiolaria, diatomaceae, and sponge spicules, which must 

 have existed in the same waters, have either entirely disappearedj 

 or become so much altered as to be recognised with difficulty. 



We are now in a position to make a scientific use of the imagi- 

 nation to picture what the conditions must have been which 

 existed at the time this deposit of hearthstone was forming. 

 This layer of about ten feet must have been many years in the 

 course of formation. Generation after generation of sponges 

 lived at the bottom of that sea, which rolled in all probability 

 where we now are. In that sea foraminifera in countless 



o2 



