224 
THE FELSPARS. 
The course of the decomposition of these minerals is most 
generally by the washing out of the alkaline silicates, and the 
consequent formation of Kaolin when pure, or of Clay when less so. 
The China Clay industry of Devon and Cornwall is, as you are well 
aware, an extremely important one, and the processes of washing, 
settling, and drying are very interesting to witness, not least perhaps 
as proving the great length of time that fine particles may remain 
suspended in water, and the beautiful green colour which is produced 
by the multitudinous reflections from them while so suspended. In 
some other cases the decomposition of felspar seems to have resulted 
in its complete removal—e./;., in the so-called Serpentine of Clicker 
Tor, by Menheniot, near Liskeard, the original presence of felspar is 
proved by its still existing where it was completely inclosed by the 
Augite, which is very little, if at all, changed, and by the forms of 
the spaces where it indented this mineral, but in the Serpentine of the 
rock there is no trace of it at all. In the change of the Olivine to 
Serpentine the felspar has utterly disappeared. 
In other cases Zeolites and Potash Mica, with the Tridymite form 
of Silica, result from the decomposition of Orthoclase, while in the 
case of the Lime felspars, as Labradorite and Anorthite, the curious 
aggregate called Saussurite, or False Jade, is perhaps as common as 
any form of alteration product. The analyses show that there is 
very often a percentage of ferrous oxide or magnesia, which can only 
be accounted for by supposing a simultaneous alteration of the 
Pyroxene or Horneblende associated with it, and a mixture of the 
products. 
I have omitted a good many points which would have been 
of interest if they could have been properly exhibited to you. In 
particular, I should have liked to have spoken rather more of the 
different twinning systems prevalent, and of the compound twinnings 
which produce such curious appearances of gratings in. polarised light. 
The minute crystals, too, in some Obsidians, which have some claim to 
be thought incipient Orthoclase, and the curious structural peculiarities 
of some of the massive forms, but this paper, although but superficial 
and cursory in the treatment of the subject, has already extended to 
quite as great a length as I can reasonably ask you to listen to, so I 
will conclude by referring those who wish to inquire further into the 
subject to— 
Geikie’s Text-book of Geology. 
Green’s Physical Geology, where there is a good resume of the optical 
properties of the various felspars. 
Kutley’s Study of Rocks. 
Bauerman’s Systematic Mineralogy, for some information on the 
crystallography and twinning. 
Zirkel’s Die Mikroskopische Beschaffenheit der Mineralien und 
Gesteine. 
Rosenbusch’s Mikroskopische Physiographie der petrographisch 
wichtigen Mineralien, 
