FAUNA AND AGE OF COATZACOALCOS FORMATION. 25 
Of the fossils found, 34 per cent are not known to be living forms. This 
feature causes Dr Dall to regard the formation as being of the late Mio- 
cene or Pliocene period, but the collection represents an off-shore or deep- 
water formation. As the character of the living fauna of the deep waters 
of the gulf of Mexico are only partly known, he suggests that the fos- 
sils collected represent a much larger percentage of living forms than 
those named in the list. 
This Coatzacoalcos formation is a most valuable discovery in the Cen- 
tral American and West Indian region, as in no other portion of it has a 
fauna of such Plioceneappearance been made known. From the phys- 
ical standpoint the occurrence of the fauna is most important, for it 
shows that the isthmus was submerged to at least several hundred feet 
during more or less of the Pliocene if not in the late Miocene period, 
when the West Indian region and southeastern part of the North Amer- 
ican continent were elevated and subjected to a long period of denuda- 
tion, which produced the broad undulating plains formed at the base- 
levels of erosion, the remains of which now stand at various altitudes. 
Thus, from the paleontological as well asthe physical existence, it appears 
that so much of the gulf of Mexico as existed during a portion of the 
Mio-Pliocene period was connected with the Pacific ocean through the 
Tehuantepec straits. It is not improbable that there may have been 
many Pliocene connections between the Gulf and the Pacific ocean 
throughout a distance of 200 miles during the greatest submergence. 
SOME UNASSIGNED DEPOSITS OF THE COASTAL PLAIN 
At the 27-kilometer post, about 4 miles directly distant from the Gulf, 
there is a small knoll rising 15 feet, composed of dark soft clayey marl 
with an included bed of 3 feet of shale, which has a dip of 30 degrees 
northeastward. On the west side of the cut a fault occurs. Near Jaltipan 
(42-kilometer post) there are several undulations, where the low hills 
(about 170 feet above the sea) are composed of arkose, while the hollows 
between them are occupied by the Columbia (?) formation. The exact 
geological positions of these isolated formations have not been determined. 
LAFAYETTE AND CoLumBiA Formations In Mrxico 
IN NORTHERN MEXICO 
The extended investigations of Mr W J McGee have given us a sys- 
tematic knowledge of the Lafayette and Columbia formations, which are 
very widely distributed over the coastal plains of the United States.* 
*W J McGee: The Lafayette Formation, Twelfth Ann. Rep. U. S. Geological Survey, 1892, pp. 
347-521, 
