By A. J. Jukes-Browne, B.A., F.G.S8. 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 
