224 bulletin: museum of compakative zoology. 



the clays are the oxidized residuum of the more pulverulent ejecta. The 

 older layers of this debris 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 prohably the case with the other great volcanoes, I 

 found no undoubted deposits of unindurated or unaltered volcanic ash. 

 It 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 1 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 canon 

 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 



i Op. cit., p. 131 



