Anniversary Address of the President, H.C. Sorby, F.R.S. 125 
sent subject orthoclase may be looked upon as eminently character- 
istic of granite and the associated gneiss, and labradorite of the 
basic erupted rocks. When only very slightly decomposed, frag- 
ments may usually be recognized by showing more or less distinct 
evidence of a well-marked cleavage, like Fig. 8, or, in the case of 
some species, by the compound banded structure seen with 
polarized light and an analyzer, as shown by Fig. 9. 
When very considerably decomposed, both these characters are 
lost, and the fragments may be of irregular form, and show only a 
very fine granular structure, appearing more or less dark and 
opaque by transmitted light, and white with a more or less red 
tint by reflected light. Fig. 10 is an example of such a fragment. 
When completely decomposed, felspar easily breaks up into granules 
of kaolin. These granules are usually very minute. From ;,; to 
roooo Of an inch in diameter is a common size, but many occur 
as small as soo0o- ‘Their refractive power is so nearly that of 
Canada balsam as to make it difficult to distinguish them when 
mounted in that substance, and they are seen to the greatest 
advantage when examined in water. It requires some care to 
clearly make out their form, but, as far as I have been able to 
ascertain it by making them turn about, they are not, strictly 
speaking, amorphous, nor yet minute crystals, but somewhat flat- 
tened and elongated particles, very similar to those obtained on 
reducing undecomposed felspar to a fine powder, as though the 
original crystalline structure had had a very powerful influence in 
determining the shape of the particles of the more or less pure 
hydrous silicate of alumina produced by decomposition. As an 
example of the shape of the particles, I refer to Fig. 11, which is 
supposed to show an average example of a highly magnified 
particle seen in various directions. There is, however, a consider- 
able variation in the relative length, breadth, and thickness. 
The depolarizing power of kaolin is high, and can be easily 
recognized in particles as small as y2too of an inch in diameter. 
Amongst the granules of decomposed felspar often occur small 
needle-shaped crystals, the nature of which remains to be deter- 
mined. 
The glassy sanidin of modern volcanic rocks differs very much 
from the felspars met with in granite ; but I must forbear to enter 
at length into such volcanic products, since they have so little 
direct bearing on British geology, to which I must chiefly confine 
my remarks this evening. 
Pumice.—For the same reason I need not say much respecting 
pumice, although it plays such an important part in deep ocean 
deposits. It has often no action on polarized light, being a true 
felspathic glass, in the midst of which gas or steam has been given 
