522 The American Naturalist. [June, 
to count as an early human foothold. My statement (See Hill 
Caves of Yucatan. Lippincott, Phila., 1895, p. 21) referring to the 
rocks of Yucatan as of Mesozic Age, is at variance with the recent 
observations of geologists, while Professor Holmes says on the other 
hand, (p. 18): “The massive beds of limestone of which the Peninsula 
is formed contain and are largely made up of the remains of the 
marine forms of life now flourishing, along the shores. Fossil shells 
obtained from the rocks in various parts of the country are all of living 
species and represent late Pliocene or early Plistocene times, thus 
possibly bringing the date of the elevation of Yucatan down somewhat 
near that of the reputed sinking of Atlantis, some eleven or twelve 
thousand years ago, or not far from the period that witnessed the oscilla- 
tions attending the glacial period.” Though true that the peninsular 
limestone is largely composed of existing marine forms we learn on 
Fig. 2. Examples of Terraces and Pyramids, superstructures omitted. 
closer examination that it is not entirely so, and that the shells are not 
ali modern. We find that the full list of age denoting fossil mollusca 
collected from the rocks of Yucatan by the expedition in 1891 of the 
Academy of Natural Sciences of Philadelphia (See Geol. Researches in 
Yucatan, by Prof. Angelo Heilprin, Proc. Acad. Nat. Sci. 1891. p- 
136) does not characterize the Yucatan rock as of Plistocene Age 
while the recent researches of geologists (Prof. J. W. Spencer makes 
the Niagara Gorge 32,000 years old) now tend to add to the antiquity 
of the Glacial Epoch, Professor Heilprin who conducted the Yucatan 
