How does Twain describe "Cooper's Indians"?

Mark Twain gives a scathingly negative opinion of Cooper's work, and especially of Cooper's most iconic novel, The Deerslayer, in a review essay entitled "Fenimore Cooper's Literary Offenses." This relatively short essay is available for free online at The University of Virginia archive, listed in the references below.
Twain sees Cooper's depiction of Indians as stereotyped and inaccurate, something he also attributes to Cooper's descriptions of the natural world. He attributes this to Cooper lacking observational skills. Twain states, "Cooper's eye was splendidly inaccurate. Cooper seldom saw anything correctly." Instead, Twain argues that Cooper simply relies on a small group of cliched or stereotypical devices, such as the presence of a person being revealed by the snapping of a dry twig.
According to Twain, Cooper attributes to Indians extremely keen powers of observation, but Cooper's own lack of attention to detail makes such portraits lack credibility. Twain gives the example of a trap Cooper has Indians set for travelers on a river:

Did the Indians notice that there was going to be a tight squeeze there? Did they notice that they could make money by climbing down out of that arched sapling and just stepping aboard when the ark scraped by? No, other Indians would have noticed these things, but Cooper's Indian's never notice anything. Cooper thinks they are marvelous creatures for noticing, but he was almost always in error about his Indians.

In other words, Twain is criticizing Cooper both for reproducing a myth of Indians as noble savages preternaturally in tune with nature and for doing so incompetently. The main focus of Twain's critique, though, is Cooper's imprecision in description, not the issues of racial stereotyping that would be raised by modern critics.
http://twain.lib.virginia.edu/projects/rissetto/offense.html