So~Ltas—On the Variolite and Associated Igneous Rocks. 115 
case of muscovite, but the condensation, when kaolin alone is com- 
pared with orthoclase, is scarcely less striking. 218 volumes of 
orthoclase are represented by 100 of kaolin. The porous character 
of kaolin pseudomorphs, after felspar, is, however, a familiar fact. 
When the transformation of minerals is effected under pres- 
sure, aS in dynamo-metamorphism, we should naturally expect 
those whose development is accompanied by contraction to be 
formed in preference to those in which it is accompanied by 
expansion; and it is possible that the excessive development of 
epidote in the igneous rocks of Roundwood stands connected with 
the earth-pressure, to which they have more than once been sub- 
jected. 
It only remains to mention the tuff, which is now a red slate 
with typical ‘‘ ausweichungs-clivage’’; it contains pieces of altered 
glass, fragments of felspar crystals, and patches and veins of 
chlorite. Since it occurs in company with variolitic and vesicular 
lava, one may fairly infer that the igneous rocks of Roundwood 
are situated not far from the actual site of a volcanic vent, which 
was in activity in Ordovician times, discharging steam and frag- 
mentary materials from its crater, with occasional overflows of 
lava. The close proximity of a somewhat coarsely crystalline 
diabase, once an ophitic dolerite, can scarcely be a matter of acci- 
dent, and one is led to suppose that in it we have a somewhat 
deeper-seated representative of the extruded spilite and variolite. 
