186 The American Geologist. September, 1897 
comprise the whole list. The Jurassic is less known and has been con- 
fused in part with the overlying Cretaceous with which it is perfectly 
conformable. It holds a large fauna of well known genera but of chiefly 
peculiar species. "Among those from the Upper Jurassic are some 
such as Aegoceras and StepJumoceras belonging strictly to a rather 
lower horizon." 
The Cretaceous deposits cover a wider area and are divided into 
Lower, Middle and Upper. Their comjjosition resembles that of the 
preceding in being for the most part mechanical, and showing a great 
absence of lime. They are cut in many places by eruptive rocks of latfiir 
date, Tertiary and even Quaternary, consisting of diorites, quartz-ande* 
sites, hornblende-syenites, granulites, diabases. The fauna is fairly 
rich and comprises numerous families, Rhizopoda, Anthozoa, Echinoi- 
dea, Vermes, Lamellibranchiata, Gasteropoda, Cephalopoda, Pisces, 
with one doubtful reptile. The absence of Hrachiopoda is a remark- 
able fact and no allusion is made to it in the text. Possibly it is an ac- 
cidental omission. Of the Tertiary only the Eocene and the Lower 
Miocene are present in any fullness, the middle and upper portions be- 
ing very local. The former lies chiefly along the gulf of Mexico and the 
latter along the gulf of California. A list of fossils shows, as the au- 
thor remarks, that it is not possible in all cases to draw a line between 
the Tertiary and Quaternary groups. 
The last named occupies a very large area in the upper valleys and 
central tableland. The materials are, as usual, loose and fragmentary, 
but some of them plainly indicate deposition from heated waters. 
Mexico, says our author, was in the earliest time a group of islands 
which wei'e subjected to severe erosion in Azoic days. Elevation pre- 
vented the deposition of Silurian and Devonian strata, and Mexico then 
"presented a large continental surface or collossal peninsula of North 
America stretching from northwest to southeast. The absolute lack 
of the first division of the Carboniferous authorized the belief that 
during the time when equivalent strata were deposited elsewhere the 
elevating movement continued and the geographic outline remained on 
the whole unchanged or with slight addition.'' The Triassic seems to 
have been a time of lakes and lagoons and of gradual subsidence and 
was followed by the Jurassic, "characterized by continental seas and 
deep water," which continued till mid -Cretaceous days, when "a gener- 
al elevation set in, resulting in the emergence of almost all the area 
which had been covered by the sea," until "at the beginning of the Cen- 
ozoic era the sea had abandoned all the center, parts of the south and 
north, and all the west of the country, but it still covered the northeast 
coast and southeast." Mexico then was a large peninsula with its point 
in the present "Central America,*' and of less size than now. Elevation 
in the early Eocene extended its limits to the east but part of this was 
again lost in the Miocene when an invasion on the west produced the 
gulf of California. At the end of the Miocene the peninsula of Yuca- 
tan emerged, and this elevation restored during the Pliocene, so much 
of the previous loss as to give to Mexico nearly its present form and 
size. E. w, c. 
