298 



MEXICO, CENTRAL AMERICA, WEST INDIES. 



are rarely felt at any great distance from the base of the volcanoes. But at 

 the end of the year 1888 several severe shocks, coinciding with the discharges of 

 mud and water from Poas and Irazu, damaged the buildings of the neighbouring 

 towns and overthrew some villages. A comparative study of the local seismic 

 phenomena and of the rainfall during seventeen consecutive years has led Pittier 

 to the conclusion that the return of igneous activity and of underground dis- 

 turbances is a direct consequence of the tropical rains penetrating to the caver- 

 nous recesses under the volcanoes. 



South of the igneous system the Costa Rican uplands are interrupted by the 

 vallej's of the Rio Grande de Tarcoles flowing to the Pacific, and of the Reven- 



Fig. 131. — Plateau and Volcanoes of Costa Rica. 

 Scale 1 : 1,200,000. 





X:; 



-^ \' 



H ,^3-=^ y... 



84'i. 



V/est cp ureervwich 



18 Miles. 



lazon descending to the Caribbean Sea. The sources of these streams are inter- 

 mingled about the Ochomogo Pass (1,100 feet), which, at a former geological 

 epoch, was flooded by one of the marine channels connecting the two oceans. 

 South of this depression stretches an almost unknown region of wooded uplands 

 some 8,000 square miles in extent, but apparently without any igneous cones. 

 According to the natives, Mount Herradura (Turubales), at the southern entrance 

 to the Gulf of Nicoya, has occasionally emitted some light vapours ; rumbling 

 sounds are even said to be heard at regular intervals in the interior of the moun- 

 tain, but these statements are doubted by Pittier, who denies that Herradura is a 

 volcano at all. It is connected by a lateral ridge with the Dota mountains, a 

 section of the main range traversing the isthmus midway between the two 



