G. W. Tyrrell—Petrography of South Georgia. 439 
groundmass. The latter is often abundantly spherulitic, and has 
undergone variable amounts of epidotization, a process referred to 
later. The quartz occurs in small bipyramidal crystals, the angles of 
which have been rounded by corrosion, and in which corrosion inlets 
and bays filled with groundmass are common. ‘The euhedral felspars 
include orthoclase, and a striped variety which has a refractive index 
well below that of Canada balsam and a maximum extinction angle 
of 14°. It is therefore a nearly pure albite. Many of the pheno- 
crysts are surrounded by a broad zone of felspathic material which is 
well marked off from the adjacent groundmass. ‘This is untwinned, 
extinguished uniformly, and appears to be orthoclase. It occasionally 
includes a number of minute quartz grains. 
The groundmass is entirely crystalline, and consists mainly of 
minute quartz grains with a minimum admixture of felspathic material. 
It generally contains numerous spherulites. These are well-formed 
spherical bodies consisting of radiating bundles of felspathic fibres, 
with a subordinate amount of quartz. They are frequently built 
about a nucleus consisting of a crystal of quartz or felspar or, in a few 
cases, of a graphic intergrowth between quartz and orthoclase. The 
more perfect growths give a good black cross between crossed 
nicols. .Sometimes, however, the arrangement of the fibres is 
irregular and the extinction confused. This occurs mostly when 
there is a series of coalescent bundles or sheaves surrounding the 
nucleal crystal. With the disappearance of the fibrous habit and 
spherulitic arrangement this material can be seen to pass into the 
felspathic zones described above. 
All these rocks contain abundant epidote. This mineral occurs 
‘mainly in the groundmass and phenocrysts, but rarely invades the 
spherulites. Consequently, the slides frequently show a striking 
‘eyed’ appearance of circular areas free from epidote, with the latter 
mineral dominating the concave interspaces. The epidote may also be 
- arranged in long streaks, and is clearly filling up small veins in the 
rock. In places bundles of fibrous chlorite appear along with the 
epidote. This process of epidotization is referred to in more detail 
in the next section, when more extreme products of the process are 
described. 
5. Lavaform and tuffaceous rocks of doubtful affinities and eprdosite.— 
In this group is included a series of much-altered rocks, which appear 
originally to have been lavas and tuffs. The alteration has given rise 
to epidote, quartz, and chlorite, and its final term is a rock composed 
entirely of epidote and quartz, and epzdosite. ‘The least altered rock 
of the series is a tuff from the coast between Slosareczyk Bay and 
Cape Disappointment. The angular fragments consist of a greenish 
or greyish palagonitic material, faintly depolarizing, and containing 
small phenocrysts of felspar. There are also curious spherical 
amygdales of a pale-green material with a dark ring near to, and 
concentric with, their margins. Oligoclase (ext. 6°) is the most 
abundant felspar, but orthoclase is also present. Little clots of 
epidote are to be found within each crystal, as elsewhere in the 
highly decomposed groundmass of the rock, but epidotization has 
progressed only to a very slight extent in this rock as compared with 
