Vol. 68.] GEOLOGY OF COSTA. EICA. 135 



events that will account for the great thickness and mode of 

 occTUTence of these river-deposits : — 



(1) The uplift and folding of the Atlantic Tertiary strata ; 



(2) Outbreak of igneous action in the form of lava-flows overlying and 



intrusions cutting the folded Tertiary strata ; 



(3) Erosion of the old valley of the Ilio Reventazon in the lavas and 



sediments ; and 



(4) Violent volcanic action, breaking through the pre-existing rocks and 



building up a high inland range of mountains with much coarse 

 fragmental material derived from the pre-existing rocks. 



Granting such a sequence of events, we should then have a river 

 "with a thalweg rising very steeply towards the interior, and much 

 fragmental material on the high ground ready for transport. The 

 rivers of this country are, moreover, liable to extraordinarily violent 

 flood-action, and so the result would be that the river would come 

 from the high ground charged with its fullest load of material. 

 In the lower part of the valley where the slope of the thalweg was 

 much less steep, most of this material would at once be dropped by 

 the river, gradually filling up the pre-existing valley to a certain 

 extent. After a time, when the upper slopes became less steep 

 and supplies of loose material became exhausted, the river, not 

 having its full load, would be able to start cutting down through 

 the deposits which it had previously formed, thus arriving at its 

 present state of having nearly re-excavated the old valley, leaving 

 only a fragmental terrace of the torrential deposits. The dip of 

 the deposits noted above is probably due to slight earth-movement 

 since their deposition, as the angle is rather too high for the 

 natural deposition of a well-bedded sand. 



As noted above, in the deposits of the Rio Reventazon patches 

 occur locally where the fragments are angular, instead of being 

 rounded ; and these may be attributed to admixture of scree-material 

 which has undergone little or no river-transport. All over the 

 country landslides take place on a very large scale during the wet 

 season, and with great frequency. This action has no doubt con- 

 tributed largely in certain parts to the formation of the ' Boulder 

 Clays ' : for example, in the deposits which are well exposed in the 

 railway-cuttings along the Pacific Railway between Atenas and 

 Orotina. In these cases, however, some stratified intercalations due 

 to water-action can generally be seen, associated with the angular 

 landslide deposit. 



Turning now to the third process, that of spheroidal weathering, 

 there can be little doubt that it plays an important part in the 

 formation of the ' Boulder Clays,' both alone and in conjunction 

 with landslides. 



Spheroidal weathering is an extremely common feature in 

 tropical climates, and has in many cases given rise to accumulations 

 very similar in general appearance to the river-deposits. A good 

 example of this may be seen in a roadside section between Cebadilla 

 Station on the Pacific railway-line and the Cebadilla power- 



