:330 
The American Geologist. 
May, 1896 
The arrangement of the Echinoidea, is as follows: 
Subclass Euechinoidea. 
Order Cidaroida. 
Subclass Palaeechinoidea. 
Order I. Bothriocidaroida. 
Order II. Perischoechinoida.—Melonitidse, Lepidesthidee, Arch 
seocidaridse, Lepidocentridse. 
Order III. Cystocidaroida. 
Order IV. Plesiocidaroida. 
A list of nearly fifty of the publications alluded to in the text and 
ithe references given in three other works which are named furnish al¬ 
most a complete bibliography of the Paleozoic echinoids. c. r. k. 
Expedicion Cientifica al Popocatepetl. By J. G. Aguilera and 
Ezequiel Ordonyez. (Comision Geologica Mexicana, 48 pp., 6 pis., with 
.a geological section and map ; Mexico, 1895.) This little tract, issued 
by the Geological Survey of Mexico, contains a history and a short 
•scientific account of Popocatepetl, the highest peak on the North 
American continent with the exception of its rival, Orizaba. The volcano 
is snow-clad from its summit to a hight of about 4,300 metres. The 
description of the crater is one of the most interesting parts of the 
tract, the authors having been able to spend 28 hours upon and in it. 
They describe the crater as elliptical in outline, bounded by irregular 
sloping walls, and measuring in its long axis 612 and in its short axis 
400 metres. Owing to the great irregularity in its margin the depth 
varies between 205 metres at the least to 505 metres at the greatest. 
The highest points in the rim have received distinct names,—Pico 
Major, El Portezuelo, El Espinazo del Diablo, and El Labio Inferior. 
Popocatepetl is a cone formed by an accumulation of many successive 
currents of lava, covered with fragmentary materials, stones, sand, 
/ashes, etc., and corresponds to those volcanoes called by some geologists 
■“ stratified cones.” (P. 20.) The lower or older of these currents shows 
a rock structure more granular and less lustrous than that of the later 
ones. Polarized light also reveals a crystalline development in the 
iformer which is not found in the more glassy and amorphous structure 
of the latter. From these and other facts the authors deduce the con¬ 
clusion that the history of the volcano has been marked by three stages 
which they denominate periodo cinerogeno. periodo brechogeno and 
iperiodo lavico. The earliest, the lava period, was the longest; during 
the second the ejecta consisted largely of pumice mixed at times 
with volcanic bombs,—blocks of andesite of the same nature as the 
lava; the third has supplied showers of ashes which overlie the older 
products and have been much eroded by wind and rain. These periods 
the authors correlate with the Pliocene, Pleistocene and Recent. Some 
of the earlier andesite lava flows are buried beneath beds containing 
remains of the horse and elephant, while a stream of very liquid basalt 
4rom the neighboring peak of Xitli overlies not only deposits containing 
vertebrate fossils but even human remains. 
Three kinds of eruptive matter are defined,—labradorite-basalt, 
ihypersthene-andesite and trachyte, of which the first is oldest and is 
