Which punctuation mark is used to set apart an introductory phrase

Colons. The colon is a stronger punctuation mark than the semicolon. As a result, within sentences, it is used only after a complete sentence--never after a dependent clause or phrase.

wordsworth tends to see nature as 'out there'; a presence which surrounds humans, and contributes to human life, but is different from humans. if you look at a typical wordsworth poem (there was a boy, ye knew him well ye rocks) - wordsworth talks about nature as something that needs to be learned about and understood, nature is different from us.  

if you look at a byron poem (the isles of greece is a good one) - byron looks at nature mainly for what it already means to man. wordsworth will look at a landscape and see the mountains, trees, valleys, and rivers as almost spiritual presences - wordsworth wants to get to know nature.  

byron will look at the same landscape and he will want to know who lives there, what are the villages? are there any farms or mines, is there some history?  

wordsworth looks at nature for what it is; byron is forever asking what it means.  

wordsworth thinks that humans are part of nature; byron thinks that nature is part of being human