You might consider the 1908 novel '[The House on The Borderlands][1]' by William Hope Hodgson.  That amazing story fulfils a lot of firsts. It was written during the first Edwardian flirtations with what we would consider 'real SF' as opposed to the often politically-motivated satires of Wells.  It also principally involves what today might be considered a 'soft spot' in the barriers between dimensions and times - offering the protagonist a view of a FAR future Earth which is inhabited by creatures so far removed from our own world they would be at home in Lovecraft's much later offerings.

A portal in every sense of the word.


  [1]: http://en.wikipedia.org/wiki/The_House_on_the_Borderland