QUATERNARY DEPOSITS 50l 



is lighter in color and from a boat appears decidedly like a rather coarse 

 tuff of massive structure. 



At Kissalya the tufflike stratum at the base of the 35-foot bluff rises 

 6 to 8 feet above the low-water stage of the river. The bulk of the ma- 

 terial over it is gravel, in part white, in part stained bright red. As seen 

 from a boat, the very bright red clays over the dull colored tufflike 

 stratum seem irregularly interstratified with the lower half of the gravel 

 formation. 



At Mocoring rapids, said to be about 140 miles from the mouth of the 

 river, there is the first exposure of hard bedrock seen in ascending the 

 river. The channel at low water is studded with black rocks that may 

 be basalt. Similar rock appears in the river bed several miles farther 

 upstream. The banks are Modern alluvium. Then the river swings 

 away from the vicinity of the south bluff, and at about 7 or 8 miles from 

 Kissalya it evidently reaches the north bluff. A bank about 60 or 70 

 feet high seems to consist largely of the light gray, massive tuff like ma- 

 terial. A little farther upstream a bluff' may be seen on the north side 

 of the river beyond a narrow strip of Modern alluvium; it is covered by 

 a fine forest of tall straight pines. Beyond on the north is an extensive 

 pine-clad rolling upland, the highest portions of which are probably 

 several hundred feet in elevation above the river. 



At Soulala (Eed Bank) the pine comes down on to a lower terrace, 

 whose riverward escarjjment is about 30 feet high and consists of strati- 

 fied gravel and variegated but dull clayey silts, judging from their ap- 

 pearance from a boat. They resemble the variegated silts seen farther 

 down the river and may represent an early stage of the river's floodplain. 

 The banks of gravel and brown sand that represent the Modern allu- 

 vium are about 15 to 20 feet high. Hence we have here in sight at one 

 time three terranes as follows : 



1. A dissected coastal (?) plain. 



2. An old river floodplain partly filling a broad shallow valley trenched 

 across the coastal (?) plain. 



3. The Modern river floodplain partly filling a broad shallow channel 

 trenched in the older floodplain deposit. 



Next the river takes a long reach toward the south bluff and presently 

 exposes black basalt-like rock under the Modern alluvium and in the 

 channel. It swings back against the pine-clad north bluff, and thence to 

 San Domingo the banks of Modern alluvium alternate with high banks 

 in rapid succession. At the San Domingo ranch the river has cut a 

 channel 200 to 400 feet wide through a broad undulating ridge of ando- 

 site lava with flow structure. It has a black coating and resembles the 



