HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 227 
Pecten, resembling some of the Pectinoid forms of the Tertiary. Ву 
sawing and polishing a number of specimens of limestone, they were 
found to be largely composed of the shells of Rudistes and Inoceramus, 
families characteristic of the Cretaceous period. Innumerable shells of 
oysters and indeterminate Foraminifera also occur in this limestone. 
The Rudistes and Inocerami attest the probable Cretaceous age of 
this limestone, which here occurs 4,500 feet above the ocean. So 
far as I am aware, this is the only known outerop of rocks of the 
Cretaceous period between Guatemala to the northwest and the Sierra 
Del Marta at the mouth of the Magdalena on the coast of Colombia to 
the south. Nowhere in the vast Isthmian region have the Cretaceous 
formations as yet been demonstrated. 
The hills composed of these limestones have a different topographic 
aspect from the others of the region, and apparently constitute a line of 
low foothills running east and west along the base of the Sierra Can- 
della. I traced them nearly ten miles east of San José. The occurrence 
of feldspar in this limestone as reported by Professor Wolff is an inter- 
esting point, as it attests the existence of igneous rocks in this region 
previous to the deposition of these rocks in Oretaceous time. 
Old Volcanic Rocks of the San José Bolson. — While at San José I 
made frequent excursions in the surrounding country and collected 
many specimens from the volcanic boulders constituting the older mate- 
rial of the valley, and exposed where the drainage had cut into it below 
the Savanna level, or old lacustral floor. According to Professor WolfPs 
determinations these consist of hornblende porphyrite, or andesite, mela- 
phyre, basalt, glassy hypersthene andesite; altered tuffs with fragments 
of porphyry; holocrystalline quartz porphyry, or granite porphyry ; 
melaphyre, entastite porphyrite; mica diorite porphyrite. As there 
is some reason to suppose that the San José bolson is at latest of 
Pleistocene age, these pre-existing rocks may possibly give additional 
data concerning the nature of the Tertiary eruptives. 
Proceeding eastward from San José toward Cartago the railway runs 
for a distance in the valley of the San José bolson, 
After a few miles the road begins to ascend toward the divide at Los 
Alto, which separates the San José from the Cartago valley. Between 
San José and this point the cuts of the railway are through yellow 
loams or clays which have a striking resemblance to the loess-like de- 
posits of the Northern United States. This clay apparently extended up 
the mountain sides as far as I could distinguish. As the road reaches an 
altitude of 4,500 feet the timber ceases and we enter a plain covered 
