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. The peuiusula of 

 Xicoya and Gulf of Dalce 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 Eican volcanoes 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 landscape 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 w r ith slopes deeply scored by erosion, 

 very much resembling our own Rocky Mountains. These masses are 

 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 volcanoes 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 the Alajuela, San Jose, 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 Rican topography, and upon the slopes in the surrounding 



