Crosby.] 484 [April 7, 
a large part of Europe, and showing, as I think, that our Eozoie di- 
visions persist in latitude as well as in longitude. 
The vast tract of more than half a million square miles lying be- 
tween the Orinoco and the lower Amazonas, which is designated by 
the general name of Guiana and includes French, Dutch, British, 
Venezuelan and Brazilian Guiana, is mainly composed of crystalline 
rocks. The Orinoco marks with great exactness the northern and 
western borders of the crystallines for a distance of eight hundred 
miles; while on the south, according to Mr. Derby, they are limited 
by a line drawn from the mouth of the Amazonas, in latitude 1° N., 
to the confluence of the Rio Negro and Rio Branco, between 1° and 
2° south latitude. 
This is for the most yart a region of elevated and broken land, 
being in striking contrast topographically as well as geologically with 
the broad Tertiary and alluvial plains which bound it on the 
north, west, and south. To the northward, between the Orinoco and 
the littoral cordillera of the Spanish Main, lies the Tertiary basin of 
Venezuela, which extends with the same natural boundaries, but rap- 
idly increasing breadth, first to the west and then to the south until 
it connects with the vast Tertiary plain of the Amazonas. 
South of the Amazonas there exists a still larger crystalline area, 
forming the highlands of Brazil, and extending through more than 
thirty degrees of latitude and twenty-five degrees of longitude. This 
mountainous tract is also separated from the Andean region by a 
broad expanse of newer formations; these sweep from the Amazonas, 
west of the Madeira, to the mouth of the Rio de la Plata. The late 
Prof. Hartt has shown very clearly that these great crystalline areas 
of Guiana and Brazil were above the seain the earliest Paleozoic 
times, forming the nuclei of the embryo continent; the subsequent 
development of the eastern half of South America, at least, consist- 
ing chiefly in the closing of the broad straits between these primi- 
tive masses, by the addition, in concentric zones, of the deposits of 
later ages. 
With the exception of some outflows of trap and, usually, a slight 
inclination away from the crystalline heights, the uncrystalline form- 
ations have suffered but little disturbance or alteration, the eastern 
half of South America being, in this respect, in striking contrast 
with the same part of North America. In this part of the world no 
general geological revolution has supervened since the close of 
Eozoic time; and the Paleozoic and more recent sediments have re- 
