Geol.— Vol. I.] SMITH—SAA'TA C ATA UNA ISLAND. 71 



drainage into them was principally from the tributary canons. 

 The valleys were shallow, with only a gentle grade from the 

 divide to their mouths, so that a comparatively slight sub- 

 sidence has almost completely drowned them. The divide 

 of the isthmus was at one time somewhat lower than at 

 present, the pass having been filled in to a certain extent by 

 alluvial deposits from the neighboring slopes. 



Although the harbors at the isthmus conform to the types 

 of the present stream topography, we cannot suppose that 

 the isthmus itself has been formed wholly by steam erosion 

 during the present topographic cycle. The break in the 

 continuity of the mass, which is found at this point, is too 

 sudden and complete to be considered as due to the forces 

 of erosion alone, in view of the fact that no such effect has 

 been produced in any other portion of the island. The 

 origin of the isthmus must be otherwise explained. The 

 most reasonable hypothesis is that of a local sag at this 

 point. This is borne out by the sudden change in the dip 

 of the bed of tuff and diatomaceous earth, as it approaches 

 Isthmus Cove (shown in Section A on the map). If this is 

 due to a local depression, that depression must have occurred 

 before the island had reached the highest point in its last 

 rise, and after the deposition of the tuff and shale. The 

 submarine platform at this point shows no apparent depres- 

 sion, so that any sag which there may have been must have 

 taken place before the platform was carved. 



In conclusion it must be said that the writer's work upon 

 the island was, owing to limited time, necessarily incom- 

 plete, and many details remain for future investigation. 



The writer wishes to express here his gratitude to Prof. 

 Lawson for his kindness in giving advice and assistance 

 throughout the work. Acknowledgements are also due to 

 Dr. J. C. Merriam. 



Geological Laboratory , 



University of California, Oct. ist, i8g6. 



