230 Transactions of the Boyal Microsco^pical Society. 
external crust". Anyone using only a moderately high power, and 
not paying attention to minute detail, might easily be led to con- 
clude that the centres of the black crosses were always cavities ; 
but by using a high power with excellent definition, it may be seen 
that they more commonly occur round more or less irregular 
and angular particles, like Fig. 10. As far as mere shape is con- 
cerned, these particles are like minute grains of sand. Some, 
indeed, are of more regular shape than that figured, so much 
so that it is occasionally diifficult to be seen that they are not really 
cavities filled with liquid ; and in fact there is good reason to 
believe that genuine full or empty cavities do really exist in the 
centre of black crosses. On the whole, therefore, it appears as 
though the production of the depolarizing action of the surround- 
ing amber cannot be attributed to the outward pressure of either 
cavities or solid fragments. As far as the optical characters are 
concerned, it is rather such as would result from a tension in the 
line of the radius. This, however, is quite impossible in the case 
of a cavity, and all but impossible even in the case of an enclosed 
fragment. We are therefore compelled to conclude that the 
depolarizing action of the surrounding amber is due to the same 
cause as that met with in the external crust. This would agree 
perfectly well with the fact of the black crosses being more com- 
monly associated with a certain amount of general depolarization. 
Taking everything into consideration, it appears as though, whilst 
the general mass of the resin was still somewhat plastic, a change 
took place on the external surface and round some enclosed angular 
particles and cavities, which made the material relatively so hard 
that its molecular state could be permanently affected by mechanical 
pressure. The more rapid hardening of the exterior than of the 
interior is what would most certainly occur in the case of any soft 
balsam exposed to the air ; and an increased tendency to the same 
change round enclosed angular fragments is quite in accordance 
with what is so often observed in the deposition of crystals, and 
with what so often happens where a condition of unstable chemical 
equilibrium is upset by mechanical means. It must be borne in 
mind that in the optical peculiarities due to tension or pressure, we 
very generally have evidence rather of relative than of positive 
changes of dimension, and that in the case now under consideration 
the result may have been brought about not by an increase in the 
bulk of the material on the surface or round the fragments, but by 
a decrease in the volume of the rest of the mass. This latter sup- 
position would agree with the facts already described when explain- 
ing the peculiarities of the balloon-shaped cavities. 
Besides such angular particles as shown by Fig. 10, others 
occur of much more complex character, and of doubtful nature, 
like Fig. 11. The examination of this led us to wonder whether it 
