HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 231 



Descent from the Central Basins to the Caribbean Sea through 

 the Canon of the Reventazon. 



From the city of Cartago to the Caribbean Sea at Port Limon the 

 railway makes a descent of 5,000 feet in less than 100 miles, ex- 

 hibiting to the observer the topography and geologic structure from 

 the great volcanic summits, nearly 12,000 feet in height, to the sea 

 level. The railway follows great barrancas marking the drainage 

 way of the river Reventazon, leading from the Cartago bolson to the 

 sea. Immediately upon leaving the Cartago valley the road threads 

 the mountain side a thousand feet above the bottom of the river gorge, 

 gradually descending the walls of this canon until it reaches the river 

 at a point still several hundred feet above sea level. From there on 

 the railway follows the grade of the stream to the coast. 



Before reading the following remarks on this subject, the reader should 

 examine the profiles and sections given on Plates VI. and VII. These 

 show the precipitous continental slope from the summit of Turialba, 

 the summit line of the canon of the Reventazon, and the line of the 

 railway, three datum lines which should be kept distinct. The canon 

 of the Reventazon (Plate X.) is comparatively a new topographic feature, 

 the elevation of the region being shown in its precipitous character. 

 With a few exceptions, to be mentioned, the great section it exposes is 

 composed almost entirely of the older boulder formation and later vol- 

 canic material. A few miles out of Cartago the road continues in the 

 boulder clay. Occasionally the railway cuts have penetrated through 

 the superficial oxidized portion of the clay, revealing a white sub- 

 structure. Some places show these clays resting unconformably upon a 

 foundation of the massive igneous rocks and tufas. 



Beyond Santiago the road tunnels through tongue-like salients pro- 

 jecting out from Irazu and Turialba, composed of more recent volcanic 

 material than has been encountered before, comprising many white and 

 pinkish colored igneous rocks of loose scoriaceous texture. One of 

 these tongues of the later eruptives at Santiago once dammed up the 

 valley into a temporary basin, and the later gorge, cut through it, is 

 clearly distinguishable. 



At Las Mesas there are other cuts through the lava, and the river 

 flows through a deep gorge. We now reach, in our descent, the basal 

 portions of the great volcanic platform, the structure and composition 

 of which are well exposed by erosion. 



Just before reaching Juanvinas the road cuts through a neck of 



