iZCf 



of the so-called Malm or Firestone Honk. 10 '' 



The rock indeed appears to have been iiuiformly regarded as a 

 siliceous or siliceo-calcareous rock of inorganic origin until about 

 two years since, when the writer, suspecting from the descriptions 

 given of it that the silica in it might have been due to sponges, 

 visited the quarries at Merstham, and there met with abundant 

 evidence of the fact. 



Beds of rock largely formed of the debris of the skeletons of 

 siliceous sponges also occur in Upper Green sand strata in other 

 areas, but in none of these does the rock assume the peculiar 

 greyish appearance and comparatively soft and porous structure 

 which characterise the malm and firestone of the northern 

 margin of the Wealdeu area. This distinctive appearance and 

 structure seem, to a certain extent at least, to be due to the 

 fact that the silica of these sponge-spicules and of the rock- 

 matrix derived from them, is principally in the colloid or non- 

 crystalline state, and has assumed the form of minute translucent 

 discs or spherules which appear to be only lightly cemented 

 together. The reasons why the silica should have been preserved 

 in this form instead of the more usual crypto-crystalline condition 

 of chert are not very clear, since to all appearance nearly 

 similar conditions of deposition appear to have prevailed in both 

 areas. 



In the hard beds of rock forming the summit of the Undercliflf 

 at Ventnor and to the west of it, in the Isle of Wight, — which, 

 like the Malm at Merstham, are due to the remains of sponges, — 

 the silica has passed into a semi-translucent chert, which, as 

 shown by its tints between crossed Nicols, is either in the state of 

 chalcedony or has even passed into the crystalline condition of 

 quartz. In this chert the sponge-spicules can as a rule only be 

 distinguished by mere shadowy outlines, or merely by the infilled 

 casts of their axial canals in glauconite or chalcedony which have 

 remained after the outer walls of the spicules have been dissolved. 

 These semi-translucent chert-bands have generally an outer 

 crust of white porous rock, harsh to the feel, which gradually 

 shades off into the solid chert. The jDorosity is due to the empty 

 casts of spicules and the silica derived from their solution has 

 gone to form the solid chert. 



The contrast between this hard chert and the Malm rock of 

 Merstham is strikingly apparent, but there is no doubt of the 

 fact that both kinds of rock originate from similar materials. 

 The proof of this has been afforded by the occurrence of cavities 

 or pockets in the chert, which are filled with the spicules of 

 sponges quite loose from each other, and in some instances 

 nearly as perfect as if they had been obtained fi-om living sponges. 

 Some of the spicules in this chert of the Isle of Wight, and at 

 Warminster, in Wiltshire, belong to the same genera, if not to 

 the same species, as those in the malm rock, but as a rule the 



