DIKES OF PALAGONIA. 
531 
Fig. 601. 
h. Laminated clay and sand. 
c. The same altered. 
from east to west, nearly divides this larger island into 
two parts, and lays open its internal structure. In the 
section thus exhibited, a dike of lava is seen, first cutting 
through an older mass of lava, and then penetrating the su¬ 
perincumbent tertiary strata. 
In one place the lava ramifies 
and terminates in thin veins, 
from a few feet to a few inches 
in thickness (see Fig. 601). 
The arenaceous laminae are 
much hardened at the point of 
contact, and the clays are con¬ 
verted into siliceous schist. In 
this island the altered 'rocks as¬ 
sume a honey-comb structure on 
their weathered surface, singu¬ 
larly contrasted with the smooth 
1 .T 1 • 1 Newer pliocene strata invaded by lava, 
and even outline which the same isle of Cyclops (horizontal section). 
beds present in their usual soft Kava 
and yielding state. The pores 
of the lava are sometimes coated, or entirely filled witli 
carbonate of lime, and with a zeolite resembling analcime, 
which has been called cyclopite. The latter mineral has 
also been found in small fissures traversing^ the altered 
marl, showing that the same cause which introduced the 
minerals into the cavities of the lava, whether we suppose 
sublimation or aqueous infiltration, conveyed it also into the 
open rents of the contiguous sedimentary strata. 
Dikes of Patagonia, —Dikes of vesicular and amygdaloid- 
al lava are also seen traversing marine tuff or peperino, west 
of Palagonia, some of the pores of the lava being empty, 
while others are filled with carbonate of lime. In such cases 
we may suppose the tuff to have resulted from showers of 
volcanic sand and scoriae, together with fragments of lime¬ 
stone, thrown out by a submarine explosion, similar to that 
Avhich gave rise to Graham Island in 1831. When the mass 
was, to a certain degree, consolidated, it may have been rent 
open, so that the lava ascended through fissures, the walls 
of which were perfectly even and parallel. In one case, af¬ 
ter the melted matter that filled the rent (Fig. 602) had 
cooled down, it must have been fractured and shifted hori¬ 
zontally by a lateral movement. 
In the second figure (Fig. 603), the lava has more the ap¬ 
pearance of a vein, which forced its way through the pepe¬ 
rino. It is highly probable that similar appearances would 
be seen, if we could examine the floor of the sea in that part 
