REVIEWS 665 
likewise be grouped in seven categories. ‘There are (r) the foundation 
rocks of ancient quartzites, serpentines, jadeite, and granite, probably 
pre-Cretaceous; (2) limestones, which are believed to be Cretaceous ; 
(3) basic igneous rocks of Eocene, and possibly of late Miocene age ; 
these rocks conceal most of the older formations and are, in turn, 
largely buried beneath later volcanics; (4) the marine Tertiary sedi- 
ments, Eocene to Pliocene inclusive, of the Caribbean side; the early 
beds of this group are extensively disturbed, elevated and broken 
through by igneous protrusions; against these deformed and eroded 
strata the Pliocene beds rest unconformably ; (5) a line of volcanoes 
surmounting the Sierras ; (6) the Pleistocene sediments of the coasts, 
and (7) the ‘‘bolsons,” base-leveled plains, benches, canyons and other 
topographic features. 
In this section the “bowlder clays” of tropical America are 
encountered. The study of some of the craters of the region throws 
some light on the origin of these clays. From the craters fine 
material, such as scoriaceous ash, is thrown in quantity. With these 
fine products are also many large bowlders of black, massive igneous 
rock. Subjected to prolonged decay, this mixture of fine and coarse 
materials would, it is affirmed, leave a residuum identical in appear- 
ance with the bowlder clays. The glacial hypothesis, as an explana- 
tion of this formation, is regarded as untenable. 
On the slopes of one of the mountains, Irazu, between the altitudes 
of 7000 and gsoo feet, there is found a fine pulverulent yellow dust, 
which is “in every way identical in lithological appearance and 
behavior with the loess deposits . . . . of the Missouri and Ohio.” 
This formation is composed of the minerals common to the lavas of 
the region, but its mode of accumulation is not discussed. 
Comparing the two sections, it is stated that “the Panama section 
is across an old land now nearly graded to the sea, where vulcanism has 
been quiescent since the Tertiary time,” while “ the Costa Rican section 
presents us a view of an ever-growing land where volcanoes have con 
tinued to pile their débris from Cretaceous time to the present.” 
Under the caption, “ The Union of the Continents and the Prob- 
? 
the general relations of the Central American 
and Isthmian regions are discussed. The meager knowledge at hand 
lems of the Straits,’ 
indicates that “previous to the vast accumulations of more modern 
igneous and sédimentary rocks of Tertiary and post-Tertiary age, a 
foundation of granitic rocks, occurring in east and west arrangement, 
