This is a picture of the early days of mountainbiking, sent to me by Gary Fisher (thanks, Gary). The bike is a Schwinn Excelsior, a "camelback" frame from before WWII. Gary and his friends discovered a pile of these, and found they were rugged and offered good ground clearance. They downhilled them on Marin County's "Mt. Tam", near San Francisco. Note that Gary had already ruined his "Klunker" by adding a derailleur in the rear, enabling him to climb back up that hill. Also notice the fancy cycling duds and protective headgear!
Both Gary Fisher and Joe Breeze marketed early mountainbikes with geometries derived from the Excelsior. A classic Schwinn "cruiser", it featured a "slack" head geometry, what motorcyclists call "a lot of rake and trail". The result is a bike with awesome stability. The old cruisers turned fine, but only if you asked them to. I had the opportunity to ride an early Fisher, and loved it! Today's designs opt for "quick" steering: they are actually no more agile than a cruiser, but will dance all over a rough trail, taking your full attention to hold a line.
For a while, recently, Breezer Bikes was marketing a replica of the Excelsior, the Breezer Ignaz X, named for Ignaz Schwinn, the company founder. The Iggy was 32-pounds, chrome-moly steel, with a 7-speed hub.
You'll hear tales that the old camelback mountainbikes were "fifty pounds". I suspect they were closer to 45 pounds, after the steel fenders were removed. They used the Schwinn S-2 rim (26 x 2"). Mine uses S-7 rims (26 x 1 3/4"). Please note, these "fractional" sizes are not interchangable with modern decimal-sized tires, such as 26 x 1.75 or 26 x 2.25. Schwinn's website does carry the proper sizes, and I've also found them made by Kenda and Chen Shin.