IZS' 



48— Dr. Hinde on the Microscopic Structure 



Here and there a few Foraminifera can be distinguished, and the 

 calcareous ingredients of the rock may probably be derived from 

 these organisms. As ah-eady mentioned, some of the harder 

 portions of the rock give no indications of calcite, and appear to 

 consist nearly entirely of the siliceous spicules, together with 

 some thin scales of mica ; and from the absence of heavier 

 mechanical materials it may be assumed that this rock was 

 formed in a sea sufficiently deep to allow of the deposits at the 

 bottom to remain undisturbed. In the beds now worked for 

 hearthstones there is a certain amount of calcite and a greater 

 proportion of mica, together with some quartz-grains and granules 

 of glauconite ; but the minute spherules or discs of transparent 

 silica derived from the solutions of the sponge-spicules are 

 abundantly present. 



Under certain conditions the Malm rock occurs as a cream- 

 coloured, soft, powdery material, very light, and throughout filled 

 with minute tubes. This kind is more jjarticularly shown at 

 Farnham, and the nearest equivalent to it at Merstham is the 

 yellowish decayed rotten-stone. The cavities in it are really the 

 negative casts of spicules, that is, the spicules in it have been 

 dissolved, and only their minute impressions in the soft matrix 

 remain. This matrix is nearly entirely composed of minute 

 discs and spherules of soluble silica, and seems therefore to have 

 been derived from the solutions of the spicules. 



Beds of Malm rock more or less closely resembling those of 

 Godstone and Merstham can be traced westwards, round the 

 northern margin of the Wealden area to Selborue, ui Hampshire, 

 and they also occur at Wallingford and other places m Berkshire. 

 At Merstham the beds have a total thickness of nearly 16 ft. ; 

 near Eeigate and Betchworth Station the harder firestone beds 

 are about 10 ft. in thickness, whilst at Farnham they are reported 

 to have a total thickness of 60 ft. At Selbome, in Hampshire, 

 the classical home of the Eev. Gilbert White, sections of the 

 Malm rock are exposed having a thickness of 15 ft., and frag- 

 ments of the harder beds are abundantly shown on the surface 

 of those fields beneath which the rock crops out. The peculiar 

 mineral nature of the rock did not escape the notice of the genial 

 author of the 'Natural History of Selborne,'^ who writes of it 

 that it is but little removed from chalk in appearance, but seems 

 so far from being calcareous that it endures extreme heat. It 

 was known in his day as the white malm, in contradistinction to 

 the chloritic marl, which was called the black malm. Gilbert 

 White lived before the days of platyscopic lenses and transparent 

 microscopic rock-sections, and it is no blame to him therefore 

 that he failed to note the minute organic contents of the rock. 



1 ' Natural History of Selborne.' Bell's edition, jj. 3. 



