Two boys separated in time - what short story is this?

[tigershrike]

As a child I read a chilling short story about two boys who go out to play. One of them falls down a well/into a hole/cave. The other goes to find help. The problem is, time moves at different speeds for them when they are separated. It moves faster for the boy in the well, so that he experiences weeks (or a long time) while the other boy's time moves so slowly he cannot see other people.

I seem to remember that the other boy was unable to find help and when he returned to the well, he found the sceleton of his friend down there.

Anyone know what this story is called or who wrote it?

Thanks! :)