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Constitution of the Ichthyolites of Stromness. 281 
The organic remains chiefly occupy the upper surfaces of 
the different layers. This circumstance would seem to indi- 
cate that the cause of the death of the fish which furnished 
these organisms, coexisted with, or may have depended upon, 
the cause of the succession of the deposits ; yet these changes 
or alternations must have occurred through a very long pe- 
riod, and exercised their influence over an extended district. 
These Ichthyolites when observed scattered over the surface 
of a stratum, exhibit the appearance of detached portions of 
coal. In many cases no trace of the fins can be perceived, 
even the individuality of their scales and bones has disap- 
peared, and an imperfect outline is all that can lead the mind 
to refer the patches in question to an organic origin. At- 
tracted by this singular feature of these remains, so imper- 
fectly preserved as to form and structure, I was induced to 
examine more narrowly their actual state, for the purpose of 
ascertaining if any peculiarity in their composition or charac- 
ter could furnish any illustration. In making the attempt, 
however, considerable difficulty was experienced in obtaining 
portions of these altered organisms, sufficiently detached 
from the surrounding matrix. In the greater number of 
specimens, there is little difficulty in perceiving the existence 
of the matter of the surrounding stratum, more or less in- 
corporated with the organisms, a circumstance which must 
influence, to some extent, the analytical results. 
In the paper by Messrs Sedgwick and Murchison, the 
authors state, page 141,—‘* By chemical analysis, which 
was kindly undertaken by Mr Herschel, it appears, as might 
be expected, that the ichthyolites differ from each other 
considerably in composition. One of them gave the follow- 
ing result :— 
“ Silex, , é d . 68.1 
Alumine, E : < - iD 
Protoxide of Iron, P 3 p 10.5 
Carbonate of Lime and Magnesia, , 14.2 
100.0 
“ The proportion of magnesia is very small. The blue 
matter of the fish is phosphate of iron, and the whole stone 
» 
