136 MR. J. ROMANES ON THE [Feb. I912, 



station. This deposit consists of rounded masses of andesitic lava 

 in a brown earthy matrix. All the boulders show a very close 

 resemblance in petrological character, and the deposit has been 

 formed by the weathering- out in place of a lava-flow. 



The action applies equally to sedimentary and to igneous rocks. 

 The first stage consists of the formation of joints which divide 

 the rocks into more or less rectangular masses. Soon the corners 

 become weathered, and then shell after shell of completely weathered 

 rock is formed, until the surface-layers are represented by isolated 

 rounded boulders of fresh rock separated by a matrix of weathered 

 material. Percolating water soon destroys any trace of original 

 structure in the weathered portion, and in the final stage Ave have 

 the rounded boulders in a yellow clayey matrix. In the case of 

 the landslide deposits the same action is liable to take place, and 

 the irregular angular fragments are soon reduced to rounded 

 boulders, the weathered portion mingling with the original earthy 

 matrix. 



It is perhaps worthy of note that the river-deposits of the 

 Reventazon Valley show in places an extraordinary resemblance, 

 both in general appearance and in mode of occurrence, to some of the 

 Old-Eed-Sandstone conglomerates in this country. Por example, 

 Lauderdale, in the Southern Uplands, a pre-Old-Ked-Saudstone 

 valley, is partly filled up by conglomerate which is being re- 

 excavated by the present Eiver Leader, in precisely the same 

 manner as the early deposits of the Eio Reventazon are being cut 

 into by the present river. To push the analogy still farther, both 

 these deposits were formed immediately after periods of violent 

 earth-movement and upheaval which would naturally give rise to 

 rapid and extensive denudation. 



YIL Summary and Conclusions. 



In a paper such as this I should not feel justified in allowing 

 myself to wander ofi" into theoretical considerations. When the 

 material at my disposal has been more fully worked out, then 

 perhaps some more definite conclusions may be arrived at, but 

 in the meantime a summary of the more important facts will be 

 given. 



In the first place, as regards stratigraphy, there is no evidence 

 of the occurrence of any pre-Tertiary rocks in the area described. 

 With the exception of the San Miguel Limestone and the limestone 

 of the Avangares district, practically all the deposits appear to be 

 marine volcanic ashes. The topography and structure of the 

 Avangares district point to three distinct periods of earth-move- 

 ment, and here the fossiliferous coastal deposits appear to rest with 

 a marked unconformity upon an older calcareous formation. How- 

 ever, considering the rapidity of earth-movement and denudation 

 in this region, it is quite possible that this break, though well 



