120 ME. J. EOMANES ON THE [Feb. I9I2, 



[d) Cebadilla. 



This locality lies some 25 miles west of San Jose, on the Eio 

 Grande. A very short distance to the west the San Jose Valley 

 narrows to a magnificent gorge, with the Cerro Caudelaria on the 

 south and the Aguacate Hills on the north. At Cebadilla the river 

 flows in a deep gorge, which affords a splendid section right through 

 the great lava-flow. Unfortunately, no Tertiary or older sediments 

 are exposed below the lava, but there is instead a thick deposit of 

 river-sands and gravels ; and obviously the river is here re- 

 excavating its old course, instead of cutting through a spur of the 

 mountains — as it is doing at El Brazil. The rocks exposed in the 

 Cebadilla gorge fall naturally into the following succession : — 



(3) Lava-flow, 



(2) River-sands and gravels, 



(Ij ' Bed-rock ' or okler volcanic series. 



(1) The older volcanic rocks are exposed in but a few 

 places along the river-bed. They almost certainly form part of the 

 volcanic complex of the Aguacate Hills, through which the old river 

 cut its valley and subsequently laid down its gravels and sands. 

 These volcanic rocks have been very considerably weathered, but 

 their andesitic character is still clearly visible. 



Some bands of this series have originally been highly vesicular, 

 and these vesicles have been subsequently infilled with calcite and 

 cryptocrystalline silica, the latter often showing beautiful spherulitic 

 structure. All these rocks have been very largely replaced by 

 calcite, and no trace of ferromagnesian minerals remains. The 

 lavas are traversed by several small dykes averaging about 3 feet 

 in width, and having a general east-and-west trend ; but this is 

 very variable, as folding has evidently taken place subsequently to 

 their intrusion. These dykes are petrographically indistinguishable 

 from some of the lavas through which they have been intruded. 

 The series illustrates well the rapidity of tropical weathering : 

 during the dry season the surface for 2 or 3 inches becomes quite 

 soft and friable, and this layer is entirely removed during the 

 floods — leaving a fresh surface to be attacked. 



The surface of this old valley-floor is very uneven, and when a 

 road was cut along the side o£ the river several peaks were en- 

 countered rising up into the overlying deposits. At the junction of 

 the Bio Yirilla with the Bio Grande, where the river-level is about 

 200 feet lower than at Cebadilla, the bed-rock extends about 50 feet 

 above the present river-level, so that the grade of the old valley 

 appears to have been less steep than that of the present Bio 

 Grande. 



(2) The sands and gravels. — Besting upon the irregularly 

 eroded surface of the bed-rock is a great series of river-deposits, 

 which vary from a coarse conglomerate (containing blocks 3 or 4 

 feet in diameter) to a thinly-bedded river-sand or silt. The thickness 

 of this series naturally varies somewhat, but it attains a maximum 



