thanks to TCM i just watched this for the first time and it's quite fun. the version they aired was about 20 minutes long. i'm not sure how long it ran originally, but this is about the right length.
the dinosaurs are very cool- despite them not looking like we "know" they looked now.
a man is pestered by his nephews to tell them a story, so he talks about his journey to Slumber Mountain with his pal Joe, and how he meets the ghost of Mad Dick who shows him marvels of the past with a weird telescope.
these marvels are a brontosaurus, two triceratops, a diatryma and the awesome t rex, all animated by Willis H. O'Brien- he later did THE LOST WORLD and KING KONG. check it out if you can.