GEOLOGY OF MY GARDEN. 



sponges, and also a collection of recent sponges as they now grow, and 

 every peculiarity which can be recognized in the present period can be 

 traced in their ancient fossil prototypes. 



FIG. 32. Cup-shaped 

 sponge (Polypothecia). 



FIG. 35. Sponge in 

 chalk (Ven'ricu- 

 lites radiatus). 



FIG. 35. Cast of 

 Diaaema in flint. 



FIG. 34. Spondylus 

 spinosus. 



The law of the deposit of silex on decomposing animal matter has 

 been extensively examined by my son, Mr. A. Hutchison Smee. Some 

 organic bodies appear to silicify with ease, others with difficulty. A 

 sponge throws down silex readily. He has been able to silicify a 

 blood-corpuscle so perfectly, that when incinerated and its animal 

 matter destroyed, it showed its structure. Bones do not appear to 

 throw down silex readily. His experiments require further examina- 

 tion before the law of silicification can be absolutely determined. 



Beyond the chalk, to the south, a layer of sand comes to the surface 

 (plate 5), and beyond this again a thin layer of clay called the Gault, 

 which is of great importance, as this clay is impervious, and passing 

 underneath the chalk prevents the water from flowing out of the chalk 

 as certainly as though the whole bed of chalk were placed in a china 

 basin. This bed of clay determines the flow of the springs, and is 

 therefore geologically of much interest. Beyond this again is the Lower 

 Green-sand, a coarse sand much larger in the grain than that immedi- 

 ately above the chalk (fig. 20). Beyond this again is the Wealden clay, 

 a deep bed totally impervious to water, and which, at the Idiot Asylum 

 at Earlswood, was bored for 1,000 feet without yielding any water. 



On the top of the last bed of sand over the chalk is a layer of flints 

 of somewhat different chemical character from those in the chalk, and 

 more ferruginous. This layer comes near to the surface at Carshalton, 



