See  The Life of Mark Twain: The Early Years, 1835-1871 pages 269-71

Much as he had been galled by the deadly routine of the schoolroom and the print shop, Sam was aghast at the compromises countenanced in the competitive newspaper market of San Francisco. “Finally there was an event,” a blatant act of censorship by Barnes of one of his articles, or so Sam recalled in 1906:

Finally there was an event. One Sunday afternoon I saw some hoodlums chasing and stoning a Chinaman who was heavily laden with the weekly wash of his Christian customers, and I noticed that a policeman was observing this performance with an amused interest—nothing more. He did not interfere. I wrote up the incident with considerable warmth and holy indignation. Usually I didn’t want to [read,] in the [morning,] what I had written the night [before; it] had come from a torpid heart. But this item had come from a live one. There was fire in it, and I believed it was literature—and so I sought for it in the paper next morning with eagerness. It wasn’t there. It wasn’t there the next morning, nor the next. I went up to the [composing-room] and found it tucked away among condemned matter on the standing galley. I asked about it. The foreman said Mr. Barnes had found it in a [galley-proof] and ordered its extinction. And Mr. Barnes furnished his reasons—either to me or to the foreman, I don’t remember which; but they were commercially sound. He said that the Call [was like the New York Sun of that day: it was the washerwoman’s paper—that is, it] was the paper of the poor; it was the only [cheap] paper. It gathered its livelihood from the [poor,] and must respect their [prejudices,] or perish. The Irish were the poor. They were the stay and support of the Morning Call; without them the Morning Call could not survive a month—and they hated the Chinamen. Such an assault as I had [attempted] could rouse the whole Irish [hive,] and [seriously] damage the paper. [The Call could not afford to publish articles [criticising] the hoodlums for stoning Chinamen].

"13 June 1906: Paragraph 5," in Autobiography of Mark Twain, Volume 2. 2013

The Chinese in San Francisco at the time were certainly the targets of discrimination and the victims of persecution. In 1868, Sam alleged, he had “seen Chinamen abused and maltreated in all the mean, cowardly ways possible to the invention of a degraded nature, “but I never saw a policeman interfere in the matter and I never saw a Chinaman righted in a court of justice for wrongs thus done him.”

Sam reported in the Morning Call in August 1864 the story of a group of boys who “were locked up in a cell in the city prison” to “give them a modified conception of what they may expect if they continue to throw stones at Chinamen and engage in other evil pursuits.” He described the next month the case of a pair of thugs “charged with having pounded a Chinaman to pieces in a slaughter house.” He noted a year later that the Call had reprinted “an account of an unoffending Chinese rag-picker being set upon by a gang of boys and nearly stoned to death.” In other words, there seems to have been no absolute proscription on covering anti-Chinese violence in the newspaper, and Sam joined the chorus of protestors against anti-Chinese prejudice. But his defense of the Chinese may easily be overstated. Donnelyn Curtis and Lawrence Berkove, for example, declare that Sam was among “a distinguished and brave minority of writers who openly defended the Chinese against an overwhelming white majority that was overtly hostile toward them."

Sam was hardly as immune from anti-Chinese prejudice or as progressive in denouncing it as his advocates claim, The Chinese were “not conscience-bound in planning and perfecting ingenious contrivances for avoiding the tariff on opium,” he editorialized in the Call a month after he joined the paper. “I am not fond of Chinamen,” Sam once acknowledged, “but I am still less fond of seeing them wronged and abused.”

Sam’s correspondence in the Buffalo Express January 22, 1879 provides indications of his persistent racisim. See my chapter in Life in Buffalo