Even as late as 1967, two years after Mariner 4 had returned its rough
22 images of the Red Planet, scientists were still debating the Canals of
Mars. In the December, 1967 issue of National Geographic Magazine,
Carl Sagan's one and only article for that periodical discussed the possibility
of the Martian canals as natural features, with an aerial photo of a series
of straight natural lines on Earth to back up the concept.

It took the first orbiter of Mars with its complete coverage of the planet
to finally resolve the issue. There were canals on Mars, but they were
decidedly natural. And one of them was a huge canyon the length of
the United States!

Check out any book on Mars from the 1950s into the 1960s and you will
be hard pressed to find any illustrations that do not include the canals:

http://antwrp.gsfc.nasa.gov/apod/ap031112.html