ppp JOURNAL OF THE PLYMOUTH INSTITUTION. 
was really a vitreous lava, with porphyritic characters. In 
this case the pseudo-amygdaloids are mainly groups of felspar 
crystals, some of which are shown by polarized light to be twinned. 
From the variety no less than the general character of the rocks 
at this particular exposure, there appears reason to conclude that 
it was a centre of volcanic activity. Steatite occurs in patches. 
At Keyham there is a finely-amygdaloidal lava, in which calcite 
crystals are porphyritically developed. 
The ordinary dunstones are greenish or bluish-grey in colour, 
weathering to a rusty brown, and losing much of their compactness 
in the process. The harder kinds are often tough, and make 
excellent road-metal. The vesicular varieties are chiefly among 
the weathered section, but much of the blue compact rock is 
amygdaloidal, and the probability seems to be that in the process 
of weathering the cavities have been emptied, rather than never 
filled. 
Sections have been cut of these amygdaloidal rocks from 
Landrake, Honicknowle, and the boss at the corner of St. 
Andrew Churchyard. The vesicles in each are filled with 
calcite! There is a grey ground-mass traversed by lath-shaped 
felspar crystals, some distinctly plagioclase. Oxides of iron occur 
in little dots and patches. Pyrites, in some examples, can be seen 
with the naked eye, but there is a considerable quantity of 
magnetite, and a little ilmenite. All these examples show that 
the rock was originally vitreous, and has undergone considerable 
change. Little beside the felspathic characters and iron oxides 
is visible, as a rule, in the ground-mass. The iron oxides explain 
the ferruginous change that occurs in the process of weathering—- 
what really takes place being the conversion of anhydrous 
peroxides into hydrous. 
Another form of amygdaloidal rock is that represented by a 
section from an exposure near Whifferton. It is a dull olivaceous- 
green, with kernels of pink calcite, and under the microscope 
shows distinct fluidal structure; the yellow-green ground has a 
somewhat marbled appearance, in consequence of the occurrence 
of dark interlacing strings and patches. This is possibly a result 
of the manner in which the original base of the rock has been 
changed into chlorite. 
1 Occasionally zeolites occur, and an amygdaloid near Kitley was found to 
have its cavities largely filled with mealy zeolite. 
