142 NOTES OF A BOTANIST 
penetrated by the dykes. It is therefore considered 
that they were either of Cretaceous or Eocene age, 
probably the former. 
The preceding summary of the geological 
structure of the Lower Amazon valley, as de- 
scribed by the American geologists, enables us to 
understand the probable origin of the country. 
The highlands of Guiana and Brazil were evidently 
in existence in Archaean times, and from their 
denudation the Silurian, Devonian, and Carboni- 
ferous rocks were successively formed in the seas 
around them. The upheaval of these deposits 
must have extended across the intervening valley, 
unless the sea there was very deep, otherwise we 
should find some indications of secondary rocks 
formed during the enormous lapse of time between 
the Palaeozoic and the Upper Cretaceous formations, 
though it is possible these may exist below the 
extensive Cretaceous and Tertiary deposits. In 
either case it seems certain that the central and 
chief portions of Guiana and Brazil have been con- 
tinuously dry land since the close of the Palaeozoic 
period, while considerable portions must always 
have been above water to furnish the source of the 
early Silurian and other sedimentary rocks. During 
this whole period denudation must have been con- 
tinuously at work, the results of which are to be 
seen in the numerous isolated ranges and mountains 
of the vast Amazon -Orinooko plateau — the huge 
domes of granite or gneiss, and the great blocks 
or ridges of palaeozoic or metamorphic rocks, the 
plains around which must have been all once buried 
under vast masses of superincumbent strata many 
thousand feet thick. Denudation has reduced this 
