Yol. 68.] GEOLOGY OP COSTA EICA. 127 



probably represents a final phase of the igneous action, and in the 

 case of at least one vein warm mineral waters are still coming up 

 through the partly-filled fissure. 



Weathering in the Avangares area. — The weathering of 

 the lavas in this district shows some special features. In all cases, 

 as would be expected in a tropical climate, a large amount of 

 surface-weathering is evident, giving rise in its final phase to 

 brown clays. Below this weathered crust, however, the igneous 

 rocks are remarkably fresh. In other cases, a very difterent type 

 of alteration has affected the rocks. This decomposition has 

 apparently gone on throughout great masses of rock, and is just as 

 complete many feet below the surface as near it. These rocks are 

 represented by material which, though quite soft and friable enough 

 to be dug with a spade, yet retains very perfectly the structure of 

 the original lava. The colour is greyish, and betrays none of that 

 brown staining which is never absent when oxidation has played 

 any part in the alteration. 



This is strongly suggestive of something more than mere surface- 

 weathering, and it is much more probably due to some type of 

 pneumatolytic action which affected the rocks in question soon 

 after their consolidation : it may well represent another phase of the 

 aqueo-igneous action which led to the formation of the auriferous 

 veins. In this connexion an exposure on the southern bank of 

 the Rio Avangares just above the mines may be recorded where a 

 rock has been somewhat similarly acted on, and in it is seen a 

 large amount of well-crystallized calcite, while the rock is im- 

 pregnated with iron and a greenish-blue mineral of a chloritic 

 character. 



Ashes and sediments. —Between the Avangares Mines and 

 the coast of the Gulf of ISTicoya is a comparatively low-lying coastal 

 belt of country occupied by a series of well-bedded ashy sediments, 

 which are fossiliferous in certain places ; these I have termed the 

 Manzanilla Beds. They are well exposed at Manzanilla on the 

 coast, and it is only at this place that fossils have as yet been 

 found in them. As will be mentioned later, many of these deposits 

 are composed entirely of volcanic debris, but in other cases non- 

 volcanic material contributes largely to their composition ; and, as 

 they lie in an area so well marked off from the volcanic region, I 

 have considered it best to treat them separately. The junction of 

 the volcanic series and the Manzanilla Beds is seen in the banks of 

 the Rio Avangares behind the village of Las Juntas. The actual 

 junction is extremely complex and difiicult to make out in any 

 detail, as the sediments are much broken up; and, owing to surface- 

 weathering, it is not always easy to distinguish the lavas from the 

 ashes. Along with the fracturing there is slight faulting; on the 

 whole, the evidence points to the existence of a zone of much- 

 fractured sediments, slightly baked, and more or less impregnated 

 with strings and veins of igneous rock : these are, therefore, rather 



