106 
THE GEAND OASTON DISTRICT. 
completely demolished by time and decay. The existing cones are, geologi- 
cally speaking, of very recent creation, even the oldest of them. The 
eruption of the basalts in the lava-caps goes back to an older period, when 
the country presented a different aspect from the present one. These ancient 
lavas form in reality “basaltic plateaus” similar, in most respects, to those 
of the Auvergne and Vivarais Mount Trumbull itself is such a plateau 
essentially, though its horizontal dimensions are greatly shrunken in com- 
parison with their former magnitudes. The older lavas are much more 
massive, and lie in thicker sheets, than any to be found among the younger 
eruptions. Lithologically, there is no difference among them. Old and 
young seem to be of identical constitution, and, in truth, it seems as if every 
coulee on the Uinkaret might have come from one and the same vesicle. 
The point to be specially noted in connection with these older basalts 
is that they are found upon the summits of the highest tabular masses, 
while the younger basalts, as a general rule, occupy the lower surrounding 
spaces. This is admirably shown in Mr. Holmes’ panorama from Mount 
Emma (Atlas sheet No. IX). On the left is shown the tabular mass of Logan, 
which is capped with ancient basalt, without a single recent cone or lava- 
flow upon it. In the middle of the picture is Trumbull, the loftiest of all, 
crowned with its old lava-cap. On the nearest corner of the mountain there 
is a solitary crater of much younger date, sending heavy streams of basalt 
down to the plain below. The profile of this crater is also well shown 
in the wood-cut (Plate XX), where it is represented on the right-hand flank 
of the mountain as it appears from the summit of Logan. The younger age 
of this crater is betrayed at once by the fact that its lava streams are shot 
down across the abruptly scarped edges of the old lava-cap and across the 
eroded edges of the underlying strata. 
The Emma platform is also an ancient basaltic plateau. But while 
Trumbull contains only one recent crater, and Logan not even one, there 
are a dozen or more on the Emma platform. It is evident, however, that 
they are much more recent than the main lava-cap; for they have many 
streams of basalts proceeding from them, which can be easily distinguished 
from the more massive flows beneath. The streams which came from those 
cones which stand near the western brink have poured down the western 
