What type of stone forms the
uppermost layer which the
water runs across at the top
of the Niagara Falls ?

Limestone, I think.

Nope !

Yo Dog,

I've checked out a few more sites and although I never did know my rocks very well, it looks like Niagara Limestone is the uppermost layer.
http://www.niagara.edu/library/nfguides/how11.jpg

I now see the problem. That is a quite
early illustration. The top layer is
dolomite which was sometimes
referred to as "Niagara limestone".
I will call your answer correct though
because you did provide a great link.
Here is one that explains further.

http://www.lockport-ny.com/Museum/rocks.htm

I see now, Dog, your site is much more current and close normally only counts in horse shoes and grenades. Thanks for the mini education!