HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 229 
loess-like formation grow the beautiful upland forests of Costa Rica, — 
a kind of second timber belt separated by a zone of grassy plain from 
tho lower belt of forest extending to sea level. 
The specimens I collected of this loess-like material were examined 
by Mr. H. W. Turner, who says regarding it:— 
“The fine brown powder labelled ‘Loess of Mt. Irazu, Costa Rica, altitude 
7,000 feet, was first examined microscopically with an objective magnifying 225 
times. The numerous mineral particles were seen to be much discolored, and 
difficult of identification. "There were numbers of little rods, perhaps siliceous 
tests or plant epidermis. In order to clear the powder of the coloring matter 
some of it was treated with hot dilute acid. After drying, the residue was 
examined. None of the rod-like tests were seen, and although siliceous plant 
epidermis or tests would not be destroyed by acid, their absence is easily ex- 
plained because of their light weight, they being probably all removed in 
washing with the brown coloring matter. Very many of the mineral particles 
which compose the residue treated with acid were well rounded, and without 
doubt had been abraded by being rolled about. Other fragments, however, 
were angular. The most abundant minerals determined were plagioclase, 
rhombic pleochroic pyroxene, probably hypersthene, and green augite. The 
hypersthene occurred as small, more or less nearly idiomorphic prisms, and 
the augite in more rounded grains and irregular fragments. No hornblende 
was identified, but, as the amount. of the material examined was very small, it 
does not follow that it was not present. No quartz was certainly detected. 
“Two thin sections of compact lava from Costa Rica were examined. They 
were both of a hornblende-pyroxene-andesite containing both augite and rhom- 
bie pyroxene. According to Professor Hill, these specimens represent the prev- 
alent lava of the region. It may therefore be safely asserted that the brown 
powder is largely of volcanic origin, and probably came from volcanoes emit- 
ting lavas of the pyroxene-andesite type, but whether it represents a subaerial 
deposit, or was formed in a body of water, is not evident.” 
The Crater of Trazu. — After passing the altitude of 9,000 feet, 
where my aneroid ceased to be of service, to a point where I estimated 
that we were at an altitude of about 10,000 feet, the loess begins to be 
covered by the overlapping of a true cinder which had rolled down from 
the summit above. The tropical vegetation ceased abruptly with the 
loess at this point, and from thence to the summit of the crater extends 
a continuous cinder slope upon which grew only scrub oaks and a 
species of heather resembling very much the blueberry summits of 
some of our New England hills. 
The crater of Irazu has been often deseribed. It is sufficient to 
state here that it consists of a vast cinder cone nearly a mile in diam- 
VOL. XXVIII. — NO. 5. 6 
