PHYSICAL FEATUEES OF COSTA EICA. 



297 



no lavas during the historic period, although both Turialba and Irazu have dis- 

 charged vast quantities of ashes, which, under the influence of the trade winds, 

 have been deposited on their south-west slopes. 



The plateaux stretching south of the volcanic system, and eroded' on both 



sides by running waters, formerly contained lakes in their cavities. The Alajuela, 

 San José and Cartage depressions had also their lacustrine basins, which were 

 gradually emptied by the erosion of the encircling walls. 



Earthquakes are very frequent, but are seldom violent, and the vibrations 



