78 
would be caused by the irregular manner in which the rods 
get mineralised. 
It might be expected that since the rods were so neatly 
and perfectly dissolved out, the spaces might get filled up 
with carbonate of lime and reproduce the silica rod as a cal- 
careous fossil ; but although 1 have examined some hun- 
dreds of those fossils, I have not found one case of this 
nature. The only cases I have found even approaching to 
this is when the centre of the rod is dissolved it is some- 
times, as in Fig. 5, filled in with carbonate of lime. I have 
noticed that when carbonate of lime is deposited in the 
cavity where the rod lay it is highly crystalline, and could 
never be mistaken for anything organic. Fig. 6 is from 
a photograph, showing the carbonate of lime in the silica 
centre. 
As the above remarks border on a subject which has 
been discussed very extensively, I may be allowed to point 
out that they settle one half of the discussion, namely, that 
silica may be dissolved in presence of calcareous fossils; but 
the other half, namely, whether or not the spaces so left 
may be filled up with carbonate of lime so as to look like 
fossils, is still an open question. 
PHYSICAL AN I) MATHEMATICAL SECTION. 
February 25th, 1879. 
E. W. Binney, F.RS., F.G.S., in the Chair. 
“ On the Mean Temperatures of the Winters of the last 
29 Years,” by the Kev. Thomas Mackereth, F.R.A.S., &c. 
It may be considered somewhat premature to institute a 
comparison of the mean temperatures of the last 29 years 
