By A. J. Juhes-Browne, B.A., F.G.S. 321 



of Oxford, Berks, Wilts, Hants, and Surrey. When examined 

 under the microscope it is seen to consist of a variety of different 

 particles ; small grains of Quartz sand, little flakes of Mica, and 

 grains of a green mineral called Glauconite ; mingled with these, and 

 sometimes forming as much as 40 or 50 per cent, of the stone, are 

 particles of a clear glassy white substance, some of them being long 

 narrow rods with sharp points, and some being very small globular 

 or discoid bodies or lumps of such globules. 



Now the needle-like rods are recognised as the spicules which 

 occur in the skeleton of a certain class of sponges — not the 

 sponges which are familiar to everyone, but like certain sponges 

 which now live in the deeper parts of the sea, and are common at 

 the bottom of the Atlantic. These sponges do not construct a soft 

 fibrous or horny skeleton like the sponges of commerce, but secrete 

 silica from the sea water and build up a siliceous framework or 

 network which is strengthened by rods and spicules of various 

 shapes. Many of these sponges shed their spicules, just as many 

 animals shed their hairs or as trees shed their leaves ; the spicules 

 fall around them and are spread through the mud or ooze in which 

 the sponges grow, so that there is nothing surprising in the fact 

 that we sometimes find layers of stone that are almost entirely 

 composed of sponge spicules, lying as thick and close as the fir 

 needles that carpet the ground beneath a fir wood in winter time. 



With regard to the globules, they also consist of a peculiar kind of 

 silica, similar to that of the spicules, not crystalline silica such as 

 sand grains are made of, but clear colloid silica which is nearly 

 structureless like solidified gum-arabic. We do not exactly know 

 how the globules were formed, but as they always occur with the 

 spicules we infer that they are derived either from the spicules or 

 more probably from the siliceous framework of the sponges, of which 

 few other traces are found. It is known that colloid silica is a very 

 soluble form of the substance, and it is probable that some chemical 

 change has taken place, which has reduced the beautiful lace-like 

 network of the sponge-skeleton to a shapeless mass of globules and 

 globular aggregations. 



A pure Malmstone consists largely of colloid silica in the form of 



