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. The 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 considei'able light upon the origin of 

 the older boulder clays. The 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 debris 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. 



