116 Part II. Chapter ı. 
by shales, greywacke, greenstones, silicious schists and especially unfossi- 
liferous limestones”‘). HEILPRIN believes that “much of this plateau has been 
formed by a progressive and long continued accumulation of detrital material, 
representing in part the distribution products resulting from mountain destruc- 
tion and in greater part the discharges from an almost endless number of 
volcanic openings. These have, as it were, filled the original valleys to their 
tips, and it is thus upon the new surface that the more recent existing valleys 
have been imposed”. 
The history of Mexico in pre-Cretaceous times is very obscure. Probably 
it was covered by sea, as is also assumed by Neumayr, in the Jurassic, at 
least in part (Hill 1898). But it seems to be well established that in the lower 
Cretaceous (Hill 1893) almost all of Mexico was submerged from the Atlantic 
to the Pacific side. In the middle of the Cretaceous period a large part of 
Mexico became land forming a southern prolongation of the western part of 
North America, which was separated in the upper Cretaceous from the eastern. 
The mountains raised in Cretaceous times have not been disturbed by depres- 
sion, but the valleys have been filled up with detritus from them and filled 
in by volcanic action, so that a plateau has been formed which has been 
only slightly eroded because of arid conditions and because it consists of 
valleys which have no outlets, but collect in their depressions the material 
eroded from all sides. 
The highest ranges are formed mainly of plutonic and volcanic rocks, 
such as granites, syenites, diorites, mineral-bearing trachytes, basalts, porphyries, 
obsidian, pearlstone, sulphur, pumice, lavas, tufa and other recent volcanic 
discharges. The most valuable rocks are the argentiferous porphyries and 
schists of the central plateau and of Sinaloa. Horizontal and stratified rocks, 
of extremely limited extent in the south, are largely developed in the northern 
states and chalk becomes very prevalent toward the Rio Grande and Rio Gila 
valleys. The sandy desert tracts, which cover north Mexico, probably owe 
their physiographic character to this chalk and sandstone. Thus the Bolson 
de Mapimi, a vast rocky wilderness, occupies and area perhaps 50,000 square 
miles in Coahuila and parts of other states. 
b. Central America. 
The Central American states are Guatemala, Honduras, Belize, San Salva- 
dor, Nicaragua, Costa Rica and Panama. A large part of Central America 
may be said to be mountainous. The main, or central chain, the Cordilleras, 
run along the Pacific coast in a general northwest-southeast direction through 
Guatemala, San Salvador, Honduras, Nicaragua, Costa Rica and Panama. In 
Guatemala, the range keeps on the whole parallel with the Pacific coast, at a 
distance of 40 or 45 miles. Its mean elevation is about 7000 feet, (Peak of 
