

126 STRUCTURAL AND FIELD GEOLOGY 
be synchronous with the formation of the granite. Analogous 
to the drusy cavities in granite are the mineral-lined or 
mineral-filled cavities of irregular shape which are char- 
acteristic of some acid igneous rocks (rhyolite). These, it 
can hardly be doubted, are deposits from heated solutions, 
formed before the rock in which they occur had cooled.* 
Probably of similar origin (at least in some cases) are the 
zeolites, which occur so abundantly in the vapour cavities of 
certain basic rocks, as, for example, the fine drusy cavities 
of the Tertiary basalts of the Ferode Islands and Iceland. 
Among subsequent secretions, the most typical are the 
amygdules referred to above. While the formation of these 
may sometimes be almost synchronous with that of the rock 
in which they occur, there can be little doubt that in most 
cases the amygdules are of subsequent origin, the mineral 
matter having been introduced by percolating water long after 
the cellular rock had cooled and solidified. Amygdaloidal 
rocks are usually more or less decomposed, the amygdules 
consisting of material derived from the breaking up of one 
or more of the original rock-constituents, especially the 
felspars. 
The minerals of most frequent occurrence in amygdaloidal 
cavities are calcite, chalcedony (agates), quartz, zeolites, 
green-earth, etc. As siliceous secretions are more durable 
than the igneous rocks in which they occur, they are often 
found in the soils and subsoils resulting from the decom- 
position of amygdaloidal rock, and under such conditions 
they are, as already indicated, often termed geodes. 
* There can be little doubt that the mineral composition of igneous 
rocks has sometimes been greatly affected by such heated solutions. 
For example, the kaolin or china-clay worked in Cornwall consists 
simply of granite decomposed 27 s¢tu. The decomposition is obviously 
due to the action of highly heated vapours coming from the more deeply 
seated and perhaps unconsolidated, or only partially consolidated, 
portion of the plutonic mass. 
