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University of California. 



[Vol. 2. 



ashy color with brownish red spots due to iron staining. Strati- 

 fication lines there are none unless very indistinct. There are no 

 fossils, no hard iron concretions and no pebbles or other rock frag- 

 ments. It is not residuary as it occurs over quite different forma- 

 tions without change of character. 



This clay is overlaid conformably but with a sharp division, by 

 a peculiar soft conglomerate of impure, light brown limonite peb- 

 bles, brown silt and clay, forming the surface formation in many 

 parts of the plain. It is usually 6 to 8 inches thick except near 

 waterways, where it increases to several feet. Often it is distinctly 

 stratified and evidently waterlaid. 



This ash-colored clay and the fine brown conglomerate are 

 fresh-water deposits made at a time that the plain was low and liable 

 to overflow from the sluggish streams. This was previous to the 

 uplift of the plain and trenching of the present valleys and hence 

 the deposits may be considered of Middle Pleistocene age. 



In the town of Santiago, whose population is about 6,ooo, 

 nearly one-third of the paving blocks are silicified wood. This was 

 gathered from the plain in the vicinity where fragments of this 

 material and even whole logs are widely scattered on the surface. 

 North of Santiago, the plain over entire square miles has fragments 

 of carnelian and agate very abundantly distributed in the soil. 

 Numerous veins of quartz and chalcedony have been formed in the 

 sub-soil within a few feet of the surface. This epoch of silicifica- 

 tion preceded the uplift and dissection of the peneplain. 



The Aguadulce Formation. — In the vicinity of the town of 

 Aguadulce, the sea invaded the plain, leveled it off and deposited 

 over the Panama formation a thick layer of gravel and sand which 

 extends back several miles from the present seaward margin of the 

 plain, but disappears so gradually that its actual original extent is 

 hard to determine. There are a few low, broad beach ridges. The 

 deposit is a mixture of more or less water-worn pebbles mostly 

 local in origin, of coarse and fine sand, of clay and of limonite. The 

 latter is the most important constituent and in places cements the 

 gravel and sand into a dark reddish brown conglomerate or impreg- 

 nates the clay so as to make it a veritable low-grade iron ore- 

 Fragments of this impure limonite bestrew the surface and outcrop 

 in large masses. It seems to indicate esturine conditions. 



