z88i.] 
Notes. 
631 
which became depopulated by fevers as soon as the sulpliur- 
works were removed from its neighbourhood to the other side of 
the island. 
“ The Cause of the Stratification of the Flints of the Upper 
Chalk.— It has long been considered that the beds of flint in the 
upper chalk are the silicified casts of sponges, but opinions differ 
concerning the force that placed them in beds ; and while modern 
books on geology mention enormous heat, chemical precipitation, 
and other old theories as the causes of their stratification, they 
at the same time give little credence to them. Now, it seems to 
me that in the habits of sponges themselves, and in the nature 
of the formation of the chalk in which they exist, is to be found 
a quite sufficient reason of their stratification. The mode of re- 
production in sponges is for one of the inhabitants (embryos) to 
detach itself from the community and float away until it arrives 
at a place suitable for the formation of a colony, which it then 
establishes, and which, after a time, in its turn comes to ma- 
turity and sends forth its citizens with the objecft of fresh coloni- 
sation. Now, it is agreed on all hands that at the time living 
sponges existed in the chalk that substance presented precisely 
all the physical and chemical conditions of the Atlantic ooze, and 
this to such a degree that the same shells (Globigerina) that are 
most abundant in the one are most plentiful in the other. 
“ Science has shown beyond doubt that the Atlantic ooze is 
formed by a constant rain of these discarded shells falling from 
the water to the bottom. 
“ These facfts explain the stratification of flints. We will 
imagine a single sponge at the bottom of the chalk sea. It 
thrives and sends out colonists, which have not far to go, as the 
land surrounding the old city is unoccupied. Consequently six 
new cities are ranged round the old one, and thrive and grow to 
prosperity. During this time the rain of chalk has been con- 
stantly falling, but the deposit has been greater on the old and 
central city than on the colonies, because it has existed longer. 
The six sponge cities then desire to colonise. They send out 
embryos, which obviously cannot form colonies in the direction 
of the old original sponge, but start in the direction of the as yet 
unoccupied ground, forming fresh colonies, which become cities, 
and in turn commence colonisation. Thus the original sponge 
grows into a circular patch of sponges, which after a length of 
time, in consequence of the falling of the chalk rain, must be- 
come a ring of life whose vitality is greater at the outer edge, 
and which gradually fades into death towards the inner as the 
central sponges become choked and covered by the shell rain, and 
are too far from the enegetic and external ring of life to be diredtly 
colonised therefrom. 
“ As any sporule starting from a healthy sponge on the outer 
edge of this ring would find in one direction an open field for 
