A GEOLOGICAL SKETCH 
former characterised, in both areas, by loftier hills 
of horizontal strata and table-topped outline, while 
the latter are lower and often rounded, disturbed 
and penetrated by abundant dykes of trap or diorite 
often of very large size or even in intrusive layers. 
These latter hills have been more denuded, and 
those near Santarem are often thickly covered with 
the scoriaceous remains of the volcanic dykes which 
probably occur in or upon them. These volcanic 
blocks almost covering the slopes and summits of 
these hills are so overgrown with shrubs, grasses, 
and other herbaceous vegetation that the subjacent 
rock on which they rest was apparently not visible. 
But no doubt a little systematic search would dis- 
cover exposures of it. There is evidently here an 
interesting problem for the next geologist who may 
visit Santarem. It may possibly be that this 
conical " hill, so strangely covered with volcanic 
debris, may really be the fragmentary remains of 
the plug or core of one of the old Cretaceous or 
Tertiary volcanoes, the more massive and harder 
blocks of its debris having protected its more friable 
portion from complete degradation. — A. R. W.] 
Aspects of Vegetation at Santarem 
I proceed now to complete my account of the 
vegetation of the mouth of the Tapajoz, and to 
show how it was affected by the change of seasons, 
from wet to dry, in the year 1850. 
The first effect of the rains was to bring out a 
luxuriant crop of grasses — tall, rank, and succulent 
on the banks of the rivers and in swampy ground, 
slender and wiry in the groves and thickets on 
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