224 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the clays are the oxidized residuum of the more pulverulent ejecta. The 
older layers of this débris having been buried longest beneath the suc- 
cessively newer and newer accumulations, are much more oxidized and 
decayed. 
Many writers speak of a fine dust which constantly overwhelms the 
traveller in this region, as volcanic ash. Except upon the higher sum- 
mits of Irazu, as is probably the case with the other great volcanoes, I 
found no undoubted deposits of unindurated or unaltered volcanic ash. 
Tt is true, however, that while the red clays may originally have been 
largely made up of volcanic cinder, they have entirely lost their original 
character as such, and the dust is only the wind dried and blown detritus 
of the present residual clays. 
The Aguacate range of mountains is a lower line of volcanic summits, 
not exceeding 5,000 feet in height, bordering the western side of the 
main volcanic plateau which constitutes the heart of Costa Rica. From 
our ship in the Gulf of Dulce and while crossing the Pacific slope we 
could see the barren volcanic rocks of these hills, and many summits 
which resembled small parasitic craters growing from the older range. 
Between San Mateo and Alajuela the highway. climbs over this range ; 
ascending the pass over these mountains from the interior margin of 
the San Mateo peneplain, which here had altitudes according to my 
rather uncertain aneroid barometer of 920 feet to a height of 3,000 feet, 
the road constantly follows a series of zigzag courses overhanging steep 
bluffs. Numerous exposures made by the workmen in constructing this 
road everywhere revealed white and green rotting tuffs of a newer aspect 
than the boulder clays of the lower slopes, with occasional boulders of 
more massive rock. This decomposed stuff extends to the very summit 
of the range, and there is hardly an outcrop along this most favorable 
section of a massive rock in situ. According to Professor Wolff's deter- 
minations the later rocks from Aguacate summit consist of a decomposed 
trachyte lava, showing fluidal (pyroclastic) structure, with some frag- 
ments of similar lava. The whole mountain, in fact, is a mass of vol- 
canic ejecta. I could not find along the road through the pass any 
other material than the scoriaceous tuffs, but, as Attwood? has pre- 
viously noted, and as I have myself verified, the older augite andesites 
underlie the later volcanic ejecta of this range, as is shown in the cañon 
of the Rio Grande which cuts across its base. (See Fig. 22. 
From the pass at the summit of this range is seen one of the most 
instructive of geographic panoramas, Looking back from the mountain 
1 Op. cit., p. 194. 
