. 
1883.) 3 Recent Literature. 65 
be protected, and then to apply more tar. Mr. Coquillet has 
proved that the army worm produces three broods in a season, 
and hibernates in the larva state: Some army worms live as cut 
worms, never migrating, while others migrate in large armies 
from field to field, and the writer argues cogently that the migrat- 
ing worms are a distinct race, the progeny of moths, the cater- 
pillars of which lived in marshes, and acquired the habit of mi- 
grating before the annual overflows; while the sedentary worms 
are bred from moths that for many generations have lived in the 
same locality. Professor French describes a new wheat-straw 
worm (/sosoma allynit), and gives a most interesting history of 
the depredations of the boll worms. 
THE GEOLOGY oF PHILADELPHIA COUNTY, ETC.'—This is among 
the latest contributions of the Geologi¢al Survey and is introduced 
by a preface of sixteen pages (entitled a letter of transmittal) by 
Professor Lesley and ninety-four pages of Mr. Hall’s report. The 
first fourteen pages of this latter contain the general remarks of Mr. 
Hall, with a table of the order of the formations as he conceives 
them to be, and a condensed summary of his reasons for believing 
the South Valley Hill rocks and the Philadelphia and Chestnut 
Hill schists superior to the Chester Valley limestone. The suc- 
ceeding thirty-three pages are devoted to general descriptions of 
the formations and contain numerous sketches, small maps and 
sections. Following these are forty-three pages of township geol- 
ogy, which complete Mr. Hall’s part of the volume. The remain- 
ing forty-three pages are devoted to the chemical work of Dr. 
Genth and Mr. F. A. Genth, Jr. 
_ This work is an exceedingly important one because it brings to 
a head in the work of the Geological Survey of Pennsylvania a 
difference of theory which has already come to the surface in other 
parts of this country and indeed in Europe as to the relative ages 
of various groups of Palzozoic and Eozoic rocks. Professor Les- 
ey in his introduction pays a justly merited tribute to the saga- 
city of Professor John F. Frazer, of the first Geological Survey of 
Pennsylvania. ; 
e states, on what ground does not appear in the volume, that 
the serpentine which Mr. Hall traces to Bryn Mawr, does not con- 
tinue its south-westwardly course through Delaware and Chester 
counties, and asserts, that ‘we can accept the palæozoic age of 
the Philadelphia rocks with a moderately reserved confidence.” 
Mr. Hall's argument may be condensed somewhat as follows: 
er ThE Philadelphia, Manayunk and Chestnut Hill beds or 
South Valley Hill, which is equivalent to part of them, cannot be 
older than the Laurentian.” (Roger's third Belt). This will be 
generally admitted, © Aaii , 
* The ( Phil: [a co ontgomery and 
Bucks, by Chants Br Hates with analne of mocks BE. A. Gerth and FA 
Genth, Jr. Second Geological Survey of Pennsylvania, C. 6. 
VOL. XVII.—wNo, I. 5 
