HILL: GEOLOGY OF THE ISTHMUS OF PANAMA. 281 
With the preliminary report om 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, eto.). 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, 94, and 36 as undoubtedly sedimentaries, and No. 36 as a tuff. 
You will see from the notes that 19 contains fragments of voleanic 
rocks, 24 is a tuff; 20, 23, and 36 are the same. "They have puzzled 
me somewhat; but I have coneluded that they are composed of an acid 
rhyolitio 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. Тһе hand specimens and slides of both roeks 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 SJÖGREN ON THE EASTERN SEOTION or 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 eoarse 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 tho tremendous 
erosion that takes place in these latitudes, been demolished, and the 
débris 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 compaet, is another layer of lime dipping 
