192 Geology of the Bosphorus. 
It is possible then that the volcanic remains at the mouth of 
the Bosphorus belong to the present geological period, that the 
centers of eruption were already surrounded by the sea in the 
time of Strabo, and that they have since sunk below the sur- 
face. ‘This seems to me, on the whole, the most plausible 
theory in regard to the Symplegades. They were volcanic cra- 
ters that made the passage into the Black Sea extremely dan- 
gerous, and sometimes, perhaps, filled up the channel by their 
eruptions, thus giving rise before historic times to this well 
dike of beautiful dioritic porphyry more than 200 feet wide, 
which can be traced for at least half a mile, although there are 
generally stratified rocks above it, which have been modified 
texture, twisted and rolled back by this eruption. Smaller 
dikes appear at other points at Bebek, so that it is not improb- 
decomposed much more rapidly t 
it, and in different localities it may be seen in every stage ° 
strata lying directly against the porphyry. . 
It is an interesting fact in cicsaemis that the Le 
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