76 FIELD CoLuMBIAN MusrEuM—GEoLocy, VOL. I. 
upon a line of side fracturing nearly at right angles to the former. 
Popocatepetl may, however, properly be considered as situated at 
the junction of the two lines of fissures, a fact which may account for 
its superior size and activity. 
The time at which the eruptions took place by which this great 
volcanic series was formed is regarded by the same authors as 
the close’ of the Mesozoic.. This estimate is based. on the entire 
absence of remains of Tertiary age in the strata of the region. Cre- 
taceous fossils are found in greater or less abundance, and following 
these, remains of Pliocene mammals, but no relics of life of inter- 
mediate age occur. It may therefore be inferred, they con- 
clude, that the close of the Mesozoic was marked by. volcanic 
activity so widespread and long-continued, that life could not exist 
in the region, nor could it again find habitation there until stable 
conditions were re-established. If it is correct to assume that these 
eruptions took place at the close of the Mesozoic, they were contem- 
poraneous with the upturning of the Rocky Mountains and the 
great igneous eruptions of Wyoming and Montana. 
Other authors, however, regard the volcanoes as of Miocene 
origin, making them contemporaneous with the volcanoes of the 
Pacific Coast region.* 
At whatever time the volcanic activity had its beginning or 
reached its maximum, it was long-continued, and some traces of it 
yet exist. Not only is Popocatepetl in a state of partial activity, but 
also in the valley of Mexico in the soil underlying some of the lava 
outflows, notably one near San Angel which came from the volcano 
of Xitli, are found human bones and fragments of pottery, which 
prove that man was living at the time the outflow occurred. (See 
Pl, VIII.) | 
The rock of this outflow is a plagioclase basalt, now quarried for 
paving. 
-Following the cessation of volcanic activity came a change of 
climate characterized by greater precipitation. The lava flows and 
volcanic debris dammed up sections of the country to form great 
basins which became filled with water, and formed numerous and 
extensive lakes. About these lakes wandered large mammals of 
various species, whose remains are more or less frequently found in 
the lacustrine deposits of the valleys. 
Already are known species of camel, bison, llama, elephant and 
horse from the beds of the region, and two nearly complete skeletons 
of glyptodon (armadillo) have been obtained. The former greater 
*Dana, Manual of Geology, 4th Ed., p. 937. 
