Geological Results of the Earth’s Contraction. 187 
that only allow the material to escape and form dikes, and those 
great fractures from which an Etna, or a range of Chimborazos, 
has originated. We have remarked in another place,* and the 
fact is sufficiently important to be again repeated, that the absence 
of the sea is no reason for the absence of volcanoes from the in- 
terior of our continents; since this same freedom from voleanoes 
existed in the Silurian epoch, when these very continents were 
mostly under salt-water. 
VILL. Geological epochs.—This subject suggests a cause for 
the transitions marking geological epochs. The formation of the 
Appalachians was attended by rendings and emissions of heat on 
a vast scale, and the baking and crystallizing of the metamorphic 
rocks of the region, as well as the debituminizing of the mineral 
coal rendering it anthracite, are attributed by the Professors Ro- 
gers to this action. It is not a matter of surprise that there should 
have been an abrupt cessation with this event, of preéxisting forms 
of marine life. 'The period when the eflects of dislocation be- 
gan to be transferred from the oceanic areas to the continents, 
appears to have been the era of this catastrophe; and it was an 
era of similar changes in various parts of the globe. The previ- 
ond era in the geological history of this country. | 
We know not how widely the last catastrophe extended over 
* This Journal, ii Ser., ii, 353. 
