70 
which can only be made out by transparent sections.” We 
must be very careful how we accept this, for in a piece 
which I prepared from Torbole, on the Lake of Garda, a 
drawing of which is shown, we find the same plant pro- 
ducing distinctly three sizes of cells at right angles to one 
another, in the proportion 5 ; 3 ; 2. 
According to Kosanaff the form of the young is always 
orbicular. 
GiimbeL gives the average of various analyses of the 
Lithothamnium nodosum from Vienna and M. Mario. 
Lime 47*14 
Magnesia 2*66 
Alumina and Oxide of Manganese 2*55 
Phosphoric Acid 0*06 
Carbonic Acid 40*06 
Insol. in Acid 4*96 
Water and Loss 2*57 
100*00 
The chief point to notice is the large amount of magnesia, 
representing per cent carbonate of magnesia. Giimbel 
points out the important bearing the large amount of mag- 
nesia which plants and animals can take up from the water 
may have upon the question of the formation of dolomitic 
rocks. 
The fossil Lithothamnia are often much altered in colour, 
One rock on the Lake of Garda might almost be described 
as a black rock with white spots ; an infiltration having 
taken place from the outside, turning all but the central 
portion a dark color, so that each piece when broken 
through shows the centre white, the rest dark. I bring this 
before your notice to show the great care that is required 
when judging from lithological characteristics, as no one 
would at first sight, and often not without microscopical 
examination, consider such a pure white rock and one 
almost black to be composed of the same materiah 
