230 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
eter, the highest rim of which, according to Petier, is 11,350 feet above 
the sea. Within this older crater are numerous later craterlets. The 
entire crater occupies but a relatively small portion of the great moun- 
tain mass which it caps, and is apparently a later parasitic summit 
growth upon a much older mass. According to the records of the 
eruptions of Irazu, the principal material ejected in historic time has 
been hot water. Тһе ejecta constituting the crater, however, consists 
principally of scoriaceous cinder, accompanied by occasional boulders of 
black basic rock ; and were it not for these historic statements one could 
believe from its recent appearance that it had been erupted within the 
past ten years. Of the great mass of material composing the present 
crater there are only two occurrences of coherent lavas, and these con- 
stitute beds only a few feet in thickness, and were probably ejected at 
widely differing intervals. One occurs in the southern part of the oldest 
rim ; the other is a stratum exposed by erosion interbedded in the ash 
of one of the secondary craterlets. Professor Wolff pronounces both of 
these rocks basalt. A piece of augite andesite was also collected from 
the crater material. 
The crater of Turialba, the easternmost of the volcanoes, is very 
much like that of Irazu. My photographs of the latter not having 
been successful, a panoramic view of Turialba is herewith given, which 
will serve to show the general characters of both these gigantic craters. 
(Plate XIX.) 
The study of the craters throws considerable light upon the origin of 
the older boulder elays. Тһе outer rim of the mother crater of Irazu 
stands some 200 feet above the interior floor. While most of this rim 
is fine scoriaceous ash, there are also many large boulders of black mas- 
sive igneous rock which were thrown out contemporaneously with the 
cinder. These boulders may be seen to-day rolling down the steep 
slopes of the cinder-cone crater, the whole hill being striated by the 
paths they have made in their descent, while the finer material is con- 
stantly sifting down upon the boulders, and filling the interstices be- 
tween them with their substance. If this débris should be subjected 
to long continued rainfall and oxidation, to which the older clays have 
evidently been subjected, the finer cinder would decay into a matrix of 
red clay, producing a result perfectly identical in appearance with the 
older boulder clays, and the glacial hypothesis would be unnecessary to 
explain the origin of the latter. 
