327 
Under the microscope the minerals seen to be present 
are quartz and muscovite, the latter being slightly greenish 
ment of felspar has been complete. A gradational alteration 
of the aplite occurs, however, and some sections show the 
incipient greisenization of the felspar. 
The quartz of the original aplite is unchanged, but some 
secondary quartz has been introduced. 
c) The Kaolin.—At a still later stage in pneumatolysis 
kaolinization of the aplite has occurred. ne kaolin is deve- 
loped in bands along minute fissures, which may contain 
thin quartz veins, and may be ascribed essentially to the 
action ‘of superheated water at a lower temperature than 
that of greisenization. : 
e evidence of pneumatolysis of the aplite is clearly 
) 1 h 
out for other fields.(? 
The formation of muscovite from microcline is doubtless 
represented by the well-known equation :— 
(.) 3 K AISi,0,+H,O —> H, K Al, (Si0,), +K, 
SiO, +5 SiO,, and the development of kaolin by 
(i) 2 K  AÍSiO,42 H,O —- ALO, 2 SiO, 
2 H,O«K, SiO, -3 SiO,, or if CO, is regarded 
as an active agent by 
(iii) 2 K AISi,O,+2 H,0+CO, —> ALO, 2 SiO, 
2 H,O+K,CO, +4 SiO,. 
porous nature of the greisen can be explained as due to the 
weathering out of kaolin from the rock. Even so, it is possible 
on greisenization, as shown by the preceding equatio 
difficult to understand, however, if this be correct, 
" () Cf. Flett, J. S.: Memoir of Geol. Surv. Eng. and Wales, 
1909, Geology of Bodmin and St. Austell, p. 118. 
