1880. | Geology and Paleontology. 819 
equally between the Mesozoic and Tertiary, and one hundred 
and twenty-five Mammalia. 
THE GEOLOGY OF THE LOWER VALLEY OF THE DELAWARE.— 
Mr. H. C. Lewis has recently investigated this subject, and pre- 
sents us with the following résumé of his results in the Proceed- 
ings of the Philadelphia Academy: 
Forming the N. W. boundary of the Philadelphia gravel and 
brick-clay is a hill of gneiss, rising two hundred feet or more 
above the river, which may be called the Upland Terrace. It has 
a N. E. and S. W. trend, and in this vicinity is at an average dis- 
tance of five miles from the river. 
. 
ous deposit. : 
A yet more recent formation, the “ River gravel and sand, 
lies within the others and close to the river, and is made up of 
flattened pebbles composed of the rocks over which the Tiver 
flows. Upon this, in the river flats, lies a modern mud, the “ Re- 
cent Alluvium.” 
Back of the Upland Terrace, isolated patches of two surface 
deposits, more ancient than any yet described, lie upon the hills. 
These are, the “ Branchtown clay,” at a height of two hundred 
and fifty feet, containing boulders of Potsdam rocks but no traces 
of Triassic red shale or of fossiliferous pebbles; and the “ Bryn 
Mawr gravel,” which caps hills of a higher elevation, ethane 
containing boulders and pebbles of identical material with those 
of the last, is characterized by the presence of a hard iron con- 
glomerate or sandstone. This conglomerate, occurring also in 
New Jersey, and named the “ Mt. Holly Conglomerate,” 1s con- 
jectured to be of Tertiary age 
In these seven formations 
the Delaware valley. 
is written the geological history of 
