I saw a flower on this cactus, so got all excited. It's at the cactus garden at Kapi'olani Community College in Honolulu. Might it be Espostoa guintheri? I like the description at On-line Guide to the positive identification of Members of the Cactus Family where it says "They are columnar plants that mostly branch at the base to form a series of stems reminiscent of a big city skyline." Also, "Another trait of Espostoa that is seen in sexually mature stems is the flower-bearing lateral cephalium. This is an area along the sides of the stems that produce and especially dense wool and/or bristles out of which the flowers are produced." If the ID is wrong, the description is still good. The third and fourth photos are zoomed images of the second one.
It might very well be. There are many columnar cacti with cephalia, but shape, colour, spines, ribs and hair fit Espostoa guentheri, as far as I can tell.
