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 

 the lower belt of forest extending to sea level. 



The specimens I collected of this loess-like material were examined 

 by Mr. II. 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- 

 bic 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 Irazu. — 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 described. It is sufficient to 

 state here that it consists of a vast cinder cone nearly a mile in diam- 



VOL. XXVIII. — NO. 5. 6 



