228 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
with thick grasses. This grassy plain, however, is succeeded on the 
higher mountains at an altitude of 7,000 feet by another belt of forest, 
which will be described later. 
From Los Alto (altitude 4,993 feet) a beautiful view is obtained of 
the Cartago basin, which, like those of the San José and Alajuela, were 
clearly once extensive lacustral areas, and upon the adjacent sides of 
the Irazu volcano we can distinguish lines resembling terraced benches. 
The yellow pulverulent loess-like clay continues to be the chief forma- 
tion until near Cartago, where erosion has again cut down into the 
boulder clays and older intrusive igneous rocks. 
Along the slopes of Irazu mountain may be seen numerous benches, 
which, if more fully studied, might prove to represent stages in the 
ancient lacustral topography of the Cartago basin. At the lower end 
of the Cartago valley, where the river Reventazon cuts out of it, it 
can be clearly seen that a great flow of volcanic matter (whether from 
Irazu or Turialba I cannot say) once dammed back the waters toward 
Cartago. These basins were no doubt formed by the collection of 
waters in the irregularities in the surface of the volcanic topography, 
aided by dams of volcanic rocks. 
Ascent of Irazu Volcano. — From Cartago (altitude 4,784 feet) I 
made, in company with Mr. Sjógren, an ascent of the Irazu volcano, 
lying to the northward of that city, and attaining an altitude of 11,350 
feet. In making this ascent we were able to secure a good idea of the 
structure of the great accumulation of material composing the mass of 
this mountain, of which the surmounting crater, large as it is, is only a 
comparatively small summit feature. In the eastern part of the town 
of Cartago the drainage cuts down to a massive basic igneous rock. 
From Cartago to an altitude of about 7,000 feet the numerous cuts in 
the road exposed nothing but the red clays and their contained boulders 
of black basic rock. 
The “ Loess” of Irazu.— Between the altitude of 7,000 feet and 
9,500 feet we encountered a peculiar geologic formation which I have 
not seen before recorded from Costa Rica. The whole slope of the 
mountain throughout this distance is composed of an exceedingly fine 
pulverulent yellow dust, which in numerous cuts And bluffs occurs as 
a compact mass in every way identical in lithologic appearance and be- 
havior with the loess deposits such as I have seen upon the Missouri 
and Ohio, having even the characteristic vertical facies upon weather- 
ing. Not a trace of a pebble or fragment of volcanic rock could we 
find in this substance, although exposures were numerous. In this 
