FORMATIONS IN CENTRAL-EASTERN- MEXICO. Di 
an indentation in the margin of the plateau in front of the volcanoes of 
Orizaba and of Sierra Negra. The marly beds along the former road are 
here either replaced by loams and gravels (more or less volcanic) or are 
obscured by them. The plains in this district are about 50 miles wide, 
and have their surfaces only 
mospheric agents. At Soli- 
dad, about 25 miles from the 
coast, and at an altitude of , 
300 feet, there is a section M\ 
inthe river bank. Thelower Dee Soc ane CHATUIITIO: 
portionconsists ofa finelight showing the junction of the eroded basaltic rocks (B) 
colored tuffaceous deposit in padoverly ine man: 
horizontal beds from 5 to 6 feet thick. It contains only occasional small 
deposited, and forming a sort of tuffaceous accumulation, which often 
appears at the surface as far as Paso del Macho, near the foot of the 
mountains. Beyond this station there is a volcanic tuff containing 
angular stones, and some great volcanic blocks derived from volcanoes 
tain valley at Atoyac, in a lateral valley this angular volcanic debris, 
which is 10 or 12 feet thick, rests upon water-worn gravel containing 
rounded boulders, some of which are 2 feet in diameter. This lower 
gravel is superimposed on the white limestones that rest against the edge 
slightly, sculptured by at- 
MyM: 
about 40feetin heightshown | ae a 
pebbles. ‘The materials consist, more or less, of volcanic ashes water- 
on the edge of the high plateau. Just before entering the great moun- 
Bethe escarpment and which are referred to the older Tertiary period. 
Figure 5.—Section between Solidad and the Mountains. 
Showing superficial deposits of the coastal plains., A, Superficial gravel referred to the Colum- 
bia formation; B, Angular débris overlying gravel; C, The Lafayette tuffaceous lower loams un- 
eonformably beneath the Columbia, and above older Tertiary limestones. 
Overlying the very much eroded surface of tuffaceous loam in the 
river section of Solidad there is a formation of coarse loose gravels from 
5 to 10 feet thick, with pebbles as large as 8 inches in diameter. ‘The 
upper part contains occasional rounded boulders of 3 or 4 feet in length. 
The formation becomes more tuffaceous and forms much of the surface 
of the country to near the foot of the mountains. The structure is well 
shown in plate 5. These boulders are volcanic, and they were probably 
