HILL : GEOLOGY OF THE ISTHMUS OF PANAMA. 281 



"With the preliminary report oh all of your rocks, some additional 

 notes are necessary. In your letter of October 29, you speak of the 

 rock of the Culebra Summit as of importance. This is a coarse basalt 

 or melaphyre corresponding in structure to many of the thick trap 

 sheets of the Mesozoic diabases (Palisades, etc.). Its coarseness and 

 lack of pores make it probable that it is either an intrusive mass, or 

 comes from a very thick flow. In the same letter you speak of Nos. 19, 

 23, 24, and 36 as undoubtedly sedimentaries, and No. 36 as a tuff. 



You will see from the notes that 19 contains fragments of volcanic 

 rocks, 24 is a tuff; 20, 23, and 36 are the same. They have puzzled 

 me somewhat; but I have concluded that they are composed of an acid 

 rhyolitic pumice much like that found in beds in some of the Western 

 States. Also 37. 



You also say that 30 and 31 may be probably intrusive through Eocene 

 clays. The hand specimens and slides of both rocks show that they are 

 composed of fragments of volcanic rocks, or the corresponding minerals, 

 in a siliceous cement which I cannot make out as intrusive. 



Notes by Ahe Sjogren on the Eastern Section of Costa Rica. 



I have shipped to-day to your address a box containing specimens 

 and fossils collected along the line of the Costa Rican Railway from Las 

 Animas to Las Lomas. 



Enclosed you will find sketches (Plate VII.) from the railroad line 

 between Las Animas and Las Lomas. They represent as far as I could 

 make out all the stratified rocks exposed to view between the two points, 

 and the intervening gaps are mostly filled up with the coarse conglom- 

 erate and boulders in clay that you noticed on our way down to Limon. 



While not wishing to put forth any theory, I cannot help saying that 

 the general features of the geology of the section seem perfectly plain. 

 The stratified rocks have at one time been broken through by intrusions 

 of molten matter, probably accompanied by a general elevation ; the re- 

 sulting ridges or peaks of igneous rock have then, by the tremendous 

 erosion that takes place in these latitudes, been demolished, and the 

 debris is now covering the stratified rocks, leaving it exposed in only a 

 few places. 



Some details at Las Animas have puzzled me very much. Just 

 west of the stratified limestone and in the middle of the conglomerate, 

 which here is very hard and compact, is another layer of lime dipping 



