From History in English Words (1926) on--a book published when he was only twenty eight years old--Barfield has been committed to the faith that "Words may be made to disgorge the past that is bottled up inside them, as coal and wine, when we kindle or drink them, yield up their bottled sunshine" (HEW 13). Etymology is of central importance to his investigation into the evolution of consciousness.

"Evoking history from words," Barfield observes, is not an exact science, nor "an automatic diary." It is, rather, "like looking back at our own past through memory; we see it, as it were, from within."

Something has stimulated the memory--a smell, a taste, or a fragment of melody--and inner light is kindled, but we cannot tell how far that light will throw its beams. Language . . . selects incidents for preservation, not so much according to their intrinsic significance as according to the impression they happen to have made upon the national consciousness. (HEW 42)
Ralph Waldo Emerson insisted that "Every word is a fossil poem." Convinced that "the more common a word is and the simpler its meaning, the bolder very likely is the original thought which it contains and the more intense the intellectual or poetic effort which went to its making," Barfield examined the poetry of ordinary language: "In the common words we use every day the souls of past races, the thoughts and feelings of individual men stand around, not dead, but frozen into their attitudes like the courtiers in the garden of the Sleeping Beauty," he writes in History in English Words. His examination of the word "quality" may serve as an example of his etymological meditations:
the word quality is used by most educated people every day of their lives, yet in order that we should have this simple word Plato had to make the tremendous effort (it is one of the most exhausting which man is called on to exert) of turning a vague feeling into a clear thought. He invented the new word "poiotes," "whatness," as we might say, or "of-what-kindness," and Cicero translated it by the Latin "qualitas," from "qualis." Language becomes a different thing for us altogether if we can make ourselves realize, can even make ourselves feel how every time the word quality is used, say upon a label in a shop window, that creative effort made by Plato comes into play again. Nor is the acquisition of such a feeling a waste of time; for once we have made it our own, it circulates like blood through the whole of the literature and life about us. It is the kiss which brings the sleeping courtiers to life. (HEW 14-15)
Geological metaphors, it has been argued, have helped shaped the thinking of such major figures as Sigmund Freud and Claude Levi-Strauss.1 A"geologic," if you will, informs Barfield's understanding of etymology as well, as is especially apparent in this foundational passage, considering the systematic discovery of etymological inquiry in the 19th century, from History in English Words:
Until a few years ago--within the memory of men still living--very little use had been made of language itself, that is to say, of the historical forms and meanings of words as interpreters both of the past and of the workings of men's minds. It has only just begun to dawn on us that in our own language alone, not to speak of its many companions, the past history of humanity is spread out in an imperishable map, just as the history of the mineral earth lies embedded in the layers of its outer crust. But there is this difference between the record of the rocks and the secrets which are hidden in language: whereas the former can only give us a knowledge of outward, dead things--such as forgotten seas and the bodily shapes of prehistoric animals and primitive men--language has preserved for us the inner, living history of man's soul. It reveals the evolution of consciousness. (14)
Though the metaphor is less explicit, geology underpins the following passage as well.
In a word here and a word there we trace but the final stages of a vast, age-long metamorphosis from the kind of outlook which we loosely describe as "mythological" to the kind which we may describe equally loosely as "intellectual thought." To comprehend the process fully, we must build up the rest of it in the imagination, just as, from seeing a foot of cliff crumble away at Dover, we may set wings to time and call up the immemorial formation of the English Channel. (84)
Barfield finds the general neglect of etymology, the ahistoricism of most investigation into the nature of language, difficult to fathom. "It is long since," he reminds in Saving the Appearances, that "men gave up the notion that the variety of natural species and the secrets of their relation to each other can be understood apart from their history." Nevertheless, "many thinkers still seek to confine the science of language, as Linnaeans once confined botany, within a sort of network of timeless abstractions. Method, for them, is another name for classification; but that is a blind alley" (116). Barfield's own method is through-and-through historical and anchored in the evidence provided by etymology.
See in particular History in English Words, passim; "The Semantic Approach to History and the Historical Approach to the Study of Meaning: (SM, Chap. 1), "Greek Thought in English Words," "Language, Evolution of Consciousness, and the Recovery of Human Meaning."
1See Frank Sulloway, Freud: Biologist of Mind and Octavio Paz, Claude Levi-Strauss.