“But tir'd at length poor Beauty slept,
And while she rested, wearied quite,
Indifference to the dear lamp crept,
And quench'd its warm, and splendid light.”

I. 3. lines 39-40
The Bard (1757)

That grip, those claws were familiar: those jaws, those death-dealing scythes, those boots. No, it was no nightmare; it was a sad and terrible reality: A man named Fulgencio Batista had just perpetrated the appalling crime that no one had expected.
ibid, p. 90
History Will Absolve Me (October 16th, 1953)

Life a Duty, reported in Bartlett's Familiar Quotations, 10th ed. (1919). Compare: "Straight is the line of Duty, / Curved is the line of Beauty, / Follow the straight line, thou hall see / The curved line ever follow thee", William Maccall (c. 1830).

Epitaph for his daughter, Olivia Susan Clemens (1896), this is actually a slight adaptation of the poem "Annette" by Robert Richardson; more details are available at "The Poem on Susy Clemens' Headstone" http://www.twainquotes.com/headstone.html
Misattributed

Falling (l. 66–68).
The Whole Motion; Collected Poems, 1945-1992 (1992)

Act i, scene 1. Compare: "But with the morning cool reflection came", Sir Walter Scott, Chronicles of the Canongate, chap. iv. Scott also quotes this in his notes to "The Monastery", chapter iii, note 11; and with "calm" substituted for "cool" in "The Antiquary", chapter v.; and with "repentance" for "reflection" in "Rob Roy", chapter xii.
The Fair Penitent (1703)

Context: And now we are wanting everyone in every country through the education department of every government to unfold that inner content of life. It’s very simple to light the lamp is so easy. And yet that process of lighting the lamp is enough to eliminate the difficulties in the darkness.

Source: The Tin Flute (1945), P. 84