440 G. W. Tyrrell—Petrography of South Georgia. 
those described later. From the association of oligoclase and ortho- 
clase in the phenocrysts this rock may be described tentatively as 
a trachyandesite tuff. ine 
of numerous slender microlites of plagioclase felspar embedded in 
a turbid indeterminate base which doubtless represents original glass. 
The rock appears to have been an andesite glass. The groundmass 
now contains abundant chlorite and numerous areas of secondary 
quartz. Epidote is as yet confined to the large amygdales consisting 
of an intergrowth of granular epidote and chalcedonie silica. 
In the next stage the groundmass is entirely changed to epidote, 
quartz, and chlorite, but in ordinary light the ‘ ghosts’ of the original 
felspar microlites are clearly visible. The replacement of the original 
material has taken place so gradually that the form of the microlites 
has been preserved. Moreover, the new-formed quartz and epidote 
is very turbid, as it seems to have replaced the substance of the rock 
without expelling the turbid dust resulting from previous atmospheric 
weathering. Only in veins and amygdales is the epidote and quartz 
free from this turbidity. Small phenocrysts of oligoclase and ortho- 
clase are still clearly visible in one of these rocks, but are in process 
of replacement by epidote and quartz. 
The final stage of the process is a rock composed largely of epidote 
with subordinate quartz, in which only the faintest trace of the 
original texture remains. In this rock the new minerals are much 
clearer than in the one above described, as the turbid impurities, 
instead of being diffused as before, have aggregated into small clots. 
In general the epidote is euhedral towards the quartz, and’perfectly 
formed crystals are found completely enveloped in the latter mineral. 
Epidotization most frequently represents a reaction between the 
lime-soda felspars and the ferromagnesian constituents of a rock, 
doubtless by the agency of percolating solutions aided by some degree 
of pressure and heat. In rocks containing plagioclase and ferro- 
magnesian minerals the constituents of epidote are there ready-made, 
and the reaction has excess silica, which crystallizes as quartz for 
a bye-product. In the rocks above described, especially the quartz- 
felsites, it is improbable that the constituents of epidote could have 
been drawn entirely from the elements of the rock. The great 
amount of epidote in some of the quartz-felsites shows decisively that 
the mineral could not have been formed from the meagre amount of 
lime, iron, and alumina available in these rocks. The inference 
may therefore be drawn that most, if not all, of the epidotic material 
has been brought in from the outside by percolating solutions. The 
progressive epidotization described above supports this view, especially 
if it be taken in conjunction with the facts that the epidote and quartz 
have occasionally replaced groundmass and felspars without expelling 
the turbidity resulting from previous weathering, and that in open 
spaces and vesicles they are quite free from such inclusions. The 
process has been one of gradual molecular replacement, without 
1 Clarke, Data of Geochemistry, 3rd ed., Bull. 616, United States Geological 
Survey, 1916, pp. 597-8. 
oy 4 
Le 
ry, 
y 
The next rock, showing a further stage in alteration, is composed. 
