HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 227 



Pecten, resembling some of the Pectinoid forms of the Tertiary. By 

 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 outcrop 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 Jose. 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 Cretaceous time. 



Old Volcanic Rocks of the San Jose Bolson. — While at San Jose 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 Wolffs 

 determinations these consist of hornblende porphyrite, or andesite, mela- 

 phyre, basalt, glassy hypersthene andesite; altered tuffs with fragments 

 of porphyry ; noncrystalline quartz porphyry, or granite porphyry ; 

 melaphyre, entastite porphyrite ; mica diorite porphyrite. As there 

 is some reason to suppose that the San Jose 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 Jose toward Cartago the railway runs 

 for a distance in the valley of the San Jose bolson. 



After a few miles the road begins to ascend toward the divide at Los 

 Alto, which separates the San Jose from the Cartago valley. Between 

 San Jose 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 



