no 
THE GRAND CASTON DISTRICT. 
the cones, it is evident that a long period of time elapsed. The ancient 
basalts are much eroded. Not only have then’ craters been demolished, but 
the massive floods which emanated from them have been greatly wasted. 
That these lava-caps are mere remnants of masses covering originally much 
broader areas is quite apparent. Their very aspect speaks strongly in favor 
of this conclusion. The surfaces of these basalts are gray with weathering, 
and the chemical action of the atmospheric agents has penetrated deeply into 
the most massive portions. None of them have preserved any of those 
rough, inflated, ropy, scoriaceous matters which form the surface of every 
fresh outpour. Nothing is left but the most compact and solid portions of 
the lava sheets. At the borders of the tabular masses which they cover 
they end in clifts, where the thick beds of lava are suddenly cut off by the 
undermining and recession of the strata beneath them. Large gorges and 
amphitheaters are excavated into the flanks of those tables into which the 
lava-fragments have fallen as the caps were undermined. No such devas- 
tation has wasted any of the younger lavas. These are much fresher in 
appearance and often as black as coal. Only around the outermost edges 
of these sheets do we find any traces of undermining and degradation. 
Almost all of them still preserve those slaggy, spongy, and scoriaceous pro- 
ducts left by the viscous stage of cooling. 
There are many indications that a long interval of quiescence sepa- 
rated the two epochs of volcanic activity, some of which will appear in the 
sequel. Here we may mention merely that none of the eruptions hitherto 
seen appear to have taken place under circumstances which leave any doubt 
as to which of the two epochs they belong. And surely no notable masses 
of any age whatever have escaped observation. In that long interval of 
quiescence important changes took place involving a large amount of ero- 
sion and a large amount of displacement, and these changes have left their 
marks which cannot be mistaken. 
Nevertheless, the younger period of eruptions was a long one; so long 
in fact that it is a fair question whether, for purposes of convenience, we 
may not subdivide it artificially, giving to the earlier outbreaks the desig- 
nation of middle-aged eruptions and to the latest that of modern eruptions. 
But it should be done with the understanding that the most ancient of all 
