THE FELSPARS. 11 



The course of the decomposition of these minerals is most 

 generally by the washinj^ 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 tlie beautiful green colour which is produced 

 by the multitudinous reflections from them while so suspended. lu 

 some other cases the decomposition of felspar seems to have resulted 

 in its complete removal — e.f/., in tho s3-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 Hornebleude 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. 

 Rutley's Study of Rocks. 

 Bauermau's Systematic Mineralogy, for some information on the 



crystallography and twinning. 

 Zirkel's Die Mikroskopische Beschaffenheit der Mineralien und 



Gesteine. 

 Eoseubusch's Mikroskopische Physiographic der petrographisch 



wichtigen Mineralien. 



