Obsidian, pumice, and scoria have fine grained or no visible crystals because they were formed. a) (cooled fast/cooled slow) b) (inside/outside) c) (above the earth's surface/below the earth's surface) d) intrusive/extrusive)

Obsidian, pumice, and scoria have fine grained or no visible crystals because they were formed.

a) (Cooled fast/Cooled slow)

b) (inside/outside)

c) (above the Earth's surface/below the Earth's surface)

d) (intrusive/Extrusive)

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

the tricuspid valve is present between right atrium and right ventricle