58 The Philippine Journal of Science 1922 
dillera. The soil is residual, except in a few favorable places 
in valleys where it is transported. Spurs are usually of hard 
material. Angat River and all the creeks are youthful. Erosion 
has taken place in the zone of fracture and carved the earth’s 
surface to that stage of erosional cycle which may be termed 
early maturity in the andesitic ‘areas and more-advanced matu- 
rity in the limestone areas. 
To account for the physiography of this limited area, one 
first tries the idea of peneplanation, and the writer was for some 
time convinced that this was the proper interpretation of the land 
forms. The reason for suggesting the theory at this time is 
that base levels have been suggested in the plateaus of northern 
Luzon and elsewhere, and it is possible that a discussion in 
the nature of the present paper might fit into a general scheme 
where there may be several such base levels; or, on the other 
hand, it might explain certain land features by other means than 
by that of peneplanation. The six principal criteria, or evi- 
dences, of peneplanation are deep rock weathering, absence of 
ledges, absence of undrained areas, monadnocks, lack of inter- 
ruption of peneplain surface on rocks of different resistance, and 
old-age drainage. Considering these separately, it is found that: 
1. This criterion of peneplanation cannot be relied upon in 
the Tropics where weathering is normally very deep. 
2. Ledges in this district are very abundant, and are quite 
evident, even though there is the usual tropical vegetation. This 
also does not apply in the district under discussion. 
8. This section is well drained, and no undrained areas were 
noted, as might be expected. 
4. A monadnock is an erosional remnant, usually of resistant 
material, left standing during the process of a river’s reaching 
base level. No monadnocks were seen, and the idea that Mount 
Mararat might be such is not plausible, since it does not have 
the softened contours of a monadnock, but is probably a mass 
of andesite of the same age as the formation in which the river 
is to-day entrenched. 
5. This cannot be stated definitely but is based on good evidence 
that some of the ridges extant are of a hard, silicified material, 
while some are harder and more resistant than the other rocks 
and have been, and are being, eroded faster than the more dur- 
able formations, indicating an interruption of a peneplain 
surface. 
