HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 201 
DESCENT FROM THE CENTRAL BASINS TO THE ÜARIBBEAN 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 barraneas marking the drainage 
way of the river Reventazon, leading from the Cartago bolson to the 
sen, 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 cafion 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 cafion of the Reventazon, and the line of the 
railway, three datum lines which should be kept distinct. The cahon 
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- 
canie 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 
