78 MISSOURI BOTANICAL GARDEN. 
though it is rare for them to cross the greater expanse 
of water separating the central from the western islands. 
The Azores are volcanic cones forming ‘‘ the highest 
points of a great plateau-like elevation which extends for 
upwards of a thousand miles from west to east, and ap- 
pears to be continuous with a belt of shallow water stretch- 
ing to Iceland in the north, and connected probably with 
the ‘ Dolphin Rise’ to the southward — a plateau which in 
fact divides the North Atlantic longitudinally into two 
great valleys, an eastern and a western.’’* Aside from 
comparatively recent swamp diatomaceous accumulations 
which Captain F. A. Chaves informs me he has discovered 
in San Miguel, a certain amount of lignite in the same 
island, and tufa casts of existing species of higher plants, 
one island only, Sta. Maria, presents fossiliferous beds. 
These have been referred to Tertiary time. This island 
alone is devoid of comparatively recent volcanic cones. 
The islands, therefore, are of late geological formation, so 
far as their present surface is concerned. Notwithstanding 
the comparatively shallow water that, as the Challenger 
researches show, separates them from one another and 
extends out on the tableland of the mid-Atlantic, they all 
drop very abruptly from the shore line to a considerable 
depth, so that there seems to be no reason for supposing 
that they have been connected together or joined to the 
mainland within recent time.T 
At sea level, frost is unknown in the Azores, though it is 
frequently experienced on the higher grounds, where 
occasional snow or sleet falls. The summer temperature 
is not very different from that of our Atlantic seaboard in 
about the same latitude. While the sun commonly rises 
clear, the higher mountains are usually enshrouded in cloud 
during the course of the day, and even in the summer 
season mist and rain are observable almost daily in the 
higher regions, and are not infrequent at sea-level. 
-* Thompson, The Aflantic, a preliminary account of the general re- 
sults of the exploring voyage of H. M. 8. “‘ Challenger.’’ 2: 23. 
+ On the geology of the Azores, see, especially, Hartung, Die Azoren. 
Leipzig. 1860. 
