1 



242 University of California. [Vol. 2. 



formation, being found nearly throughout its extent. Although 

 there are some thin layers of fine gravel and sand, the great mass 

 of the formation is a shale, everywhere characterized by the same 

 dull greenish or olive tint. It is not hard rock, yet is sufficiently 

 lithified to be used as a building stone. It contains but little iron 

 and weathers usually into a yellow clay soil. In some places it 

 abounds in fossils which are generally imperfect and so brittle as to 

 be incapable of preservation. They are of marine species of gaster- 

 opods, lamellibranchiates and allied forms. They differ decidedly 

 from the species at present so plentiful on the Pacific beach of the 

 Isthmus, but certainly are not of such old types as the Carbonif- 

 erous or Jura-Trias. As I remember the fauna it more nearly 

 resembled the Chico than that of any other formation, but I can 

 not say that they are specifically identical. 



The Santiago formation forms the foundation of the entire 

 Aguadulce-Santiago plain, over a large portion of which it is the 

 surface formation. Here it always dips in some direction, but 

 rarely at a high angle. It seems to have been formed into low 

 folds and was perhaps faulted in places. It is extensively developed 

 in the northern half of the Peninsula of Azuero, where it forms 

 mountains. On the eastern shore of the Gulf of Montijo it can be 

 traced south until it is seen to overlie the formations previously 

 described in this paper. In this direction it loses its breccia beds, 

 becomes more sandy, better lithified and titled at a high angle, 

 even in places standing on edge. On the lower two miles of the 

 Torio River and at a promontory a short distance south of the 

 mouth of the stream, it is exposed in 'beds of hard sandstone and 

 shale over one thousand feet in thickness and dipping steeply in a 

 general westerly direction. Here it is in contact with the green 

 igneous formation, fragments of which, particularly the red chert, 

 are included in its basal breccia-conglomerate. Farther north it is 

 seen to rest un conformably on the Montijo conglomerate and 

 nearly abuts against and caps a boss of Torio limestone. 



The non-conformity at the base of the Santiago formation is of 

 the same general nature and probably the same time value as that 

 which separates the auriferous slate series and the Chico series in 

 northern California. All the preceding formations, no matter how 



