HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 225 
toward the Pacific over the course we have just travelled, every detail 
of the geography is brought out in the clearest relief. "Тһе peninsula of 
Nicoya and Gulf of Dulce and the broad waters of the Pacific can be 
clearly distinguished, while the really rough surface of the San Mateo 
peneplain, which we have been crossing since leaving Punta Arenas, 
spreads out like a level plain below us, the summits upon which we 
stand having clearly been ‘piled up above it by volcanic ejection. 
The Central Volcanic Plateau. — Looking to the eastward, however, 
the eye beholds an exquisite view of the central volcanic plateau of 
Costa Rica. This presents a complete and sudden transition from the 
scenery covered by vegetation which everywhere prevailed since I 
landed, to an open, timberless mountain and basin topography. The 
transition is as if one had been suddenly taken from the Isthmian low- 
lands and deposited into the great bolson valleys of Mexico or those 
of our own Cordilleran region. As far as I could see, the superb sum- 
mits of the Costa Rican voleanoes form a background, Poas (altitude 
8,700 feet) [Plate XIX.], Barba (altitude 9,000 feet), Irazu (altitude 
11,350 feet), and Turialba (altitude 11,300 feet), while in the foreground 
between them and our point of view, at an average altitude of 5,000 feet 
above the sea, lie the great fertile upland basin valleys of Costa Rica. 
The landseape changes in color from the deep green of the coastal 
vegetation to the grays and browns of the higher mountain scenery of 
western America.. The mountains as a whole have not the aspect of 
symmetrical cinder-cones, however, but collectively they constitute a 
long series of high, serrated masses with slopes deeply scored by erosion, 
very much resembling our own Rocky Mountains. "These masses aro 
surmounted here and there by true cinder cones, which in themselves 
form but a small proportion of the entire mass. I had anticipated seeing 
the voleanoes with great interest, but had not expected to find the mag- 
nificent types of the ancient bolson (basin) topography spread out at my 
feet like those which are such a familiar sight in the Cordilleran regions 
of Mexico and the United States. In this topography will perhaps yet 
be found important evidence concerning the evolution of the region, and 
a key to the age of the great mass of the mountains of which the present 
craters are only the surmounting finials. 
Upon the maps and profile three of these basins are indicated. These 
may be called tho Alajuela, San Josó, and Cartago bolsons, respectively. 
I regret that it was impossible to map them out accurately, for to my 
mind they are by far the most interesting and important feature in 
the Costa Riean topography, and upon the slopes in the surrounding 
