Vol. 68.J GEOLOGY OF COSTA KICA. 137 



marked, does nofc represent any very great stratigraphical gap. It 

 is probable, both from the fossils occurring in some of these beds 

 and from the fact that marine Tertiary deposits cross the Atlantic- 

 Pacific watershed between San Jose and Cartago, that there was 

 free interoceanic connexion during their deposition. 



There is no evidence on the Pacific side of any later marine 

 deposits to correspond with the Pliocene deposits of Limon, and 

 therefore a widespread submergence in late Tertiary time does 

 not seem probable. In this connexion, it is important to remember 

 that the Pliocene of the Atlantic coast apparently extends only 

 to a very trifling height above sea-level. 



As regards petrology, the most interesting fact is the evidence of 

 the occurrence of true monzonites in the Cerro Candelaria, and it 

 is possible that analyses of some o£ the associated lavas may place 

 them with the trachyandesites rather than with the true andesites, 

 since, in some, alkali-felspar is developed to a small extent. The 

 occurrence of a limburgite in the Avangares region is interesting, 

 as being a type not previously recorded from Costa Eica, and I 

 believe that it must be much older than the main mass of the 

 volcanic rocks of that region. The volcanic rocks other than this 

 fall into two clear groups : — (1) an older series of andesites and 

 ashes closely connected with the marine deposits described ; (2) the 

 range of recent volcanoes largely composed of ashes in the area 

 tinder consideration, but containing lavas in the lower parts, which 

 also appear to be pyroxene-andesites. 



As regards topography, the most interesting feature is the great 

 San Jose Yalley, and its prolongation westwards as the ' San Mateo 

 peneplain ' of Hill. 



The covering of lava, which occurs at least in the eastern part, 

 represents, I believe, the outbreak of the second great period of 

 igneous activity, and on the top of these lavas are piled the great 

 ash-cones which form the northern boundary of the valley. The 

 so-called ' lake-terraces ' near Alajuela are regarded as fault-scarps. 

 As to the southern boundary of the valley, I can say but 

 little. The lavas clearly terminate against the Cerro Candelaria, 

 but whether the relation between the mountains and the valley, on 

 this side, is of the nature of a fold or a fault, is not at present 

 clear, though what evidence there is points rather to the former. 



The ' Boulder Clays ' are attributed to three causes, torrential 

 river-action, landslides, and spheroidal weathering. 



In conclusion, I wish to express my sincere gratitude to many 

 friends who have helped me. In the first place, to Mr. W. J. 

 Le Lacheur, for his advice and help in undertaking the work; 

 to the President and Government of Costa Eica and to Mr. P. 

 Nutter Cox, British Consul in San Jose, for their unfailing kindness 

 in assisting me while in the country; and to the many friends in 

 Costa Eica, whose kindness and hospitality made this investigation 



a.J.G.S. No. 269. L 



