Boston Meeting of the Geological Society. 141 
in Pennsylvania the strata above the great Silurian limestone (in to xn 
inclusive) measure in the Pottsville basin 23,700 feet, but in Lycoming 
county they are only 9,700 feet thick. These measures being taken 
from the once level horizon of overlying coal measures, their base, the 
limestone, evidently conformed to the curved under surface of the mass 
of strata in a trough whose form is indicated by the measures of unequal 
thicknesses. A more detailed statement would show several synclines 
of deposition indicated by inequalities of thickness in Devonian or Car- 
boniferous strata, which now form the Anthracite basins. 
Strata have been folded by horizontal compression. They resist com- 
pression as struts oppose a thrust. A bent strut yields at the bend; 
strata forming a syncline of deposition yield in that syncline, which, be- 
ing folded closer, becomes a syncline of deformation. The decided dip 
from the shore seaward is in a position to be sharply upturned; it may 
become vertical or overturned. In this position it may be eroded to 
great depth without much change in the position of its outcrop, which 
remains marking approximately the ancient shore line. The application 
of this method of determining shore lines to the geography of the Ap- 
palachian province in Paleozoic time has been found fruitful and it is 
believed to be reliable. In considering the evidence relating to shore 
lines, the facts of structure considered in the light of this reasoning ap- 
pear to be as important as facts of distribution of sediments and of 
lithologic characters. 
11. An account of an expedition to the Bahamas. Alexander Ag- 
assiz, Cambridge, Mass. (Introduced by W. M. Davis.) This illus- 
trated lecture by the director of the Museum of Comparative Zoology 
at Harvard University, was given on Wednesday evening. It summa- 
rized the observations taken during cruises of nearly 3,300 miles among 
the Bahama islands in the early part of last year. A letter describing 
this exploration was in the American Journal of Science for April, 1893. 
These islands (as far as to Turk's island) are found to be all of eolian 
origin. They were formed at a time when the now submarine Bahama 
banks must have been practically one huge irregularly shaped mass of 
low land, from the beaches of which successive ranges of low dunes of 
coral sand, such as still occur in New Providence, must have originated. 
After the area was thus raised, there was an extensive gradual subsid- 
ence which can be estimated at about 300 feet, and during this subsid- 
ence the sea has little by little eaten away the eolian lands, leaving only 
here and there narrow strips in the shape of the present islands. 
12. The, Later Tertiary lacustrine formations of the West. William 
B. Scott, Princeton, N. J. The Equus beds are shown by their fauna 
to be probably Pleistocene, and are separated by a marked unconform- 
ity from the Loup Fork. The latter can now be separated into three 
horizons, characterized by decided differences in their faunas. Of these 
the oldest is the Deep River formation of Montana, which is known 
only in the valley of that river. The second, which may be called the 
Nebraska, is developed on a vast scale from South Dakota to Mexico ; 
while the third, named the Palo Duro formation, has been found over- 
lying the Nebraska in Texas and northern Kansas. The last appears to 
be the equivalent of the Archer fauna of Florida, which on stratigraph- 
ical grounds Dal! regards as Pliocene. In tabular form these horizons 
may be arranged as follows : 
Pleistocene. E(|tnis hods. 
( Peace Clerk. 
Pliocene. < Blanco. 
( Palo Duro ami Archer Hippidium beds. 
Upper Miocene. Loup Fork. < Nebraska (Josoryx beds. 
/ Dee 1 1 River Cyclopidius bedn. 
Morphological palaeontology requires that correlation of these hori 
zons with those of the old world be attempted. For this purpose the 
