The downtrodden peasants of the Campo are described as having "mute eyes"
in Chapter 1-7 ,
implying that they are powerless to do anything but observe and accept the political
events periodically sweeping over them. The Sierra, on the other hand, symbolizes
in the novel the Ideal that inspires those political passions, but which itself
is described as "a colossal embodiment of silence."
In other words, both extremes are "mute witnesses." Conrad's "first
vision" of Costaguana thus contains its entire tragic scope, in which men's
passions are ignited by dark, uncaring ideals to fall upon the powerless, suffering
underclass.