As Mark Twain learned the trade of steamboating he needed to be able to discern between the beautiful aspects of the river which he talked about figuratively and the critical awareness of those same aspects as they pertain to navigation. The two sets of details are in juxtaposition.
However, there is something larger happening in the excerpt.
What is it? Define it as the thematic concern for your paper. Develop your thesis around the larger message. In other words, what is the big picture here?
What is the larger message that the writing attempts to engage for the audience?
Most, if not all, of the language speaks about two views of the river, but the tension between those descriptions is both explicit and implicit.
What are the implied factors (implications) beyond the language of the piece? The explicit examples (tools/devices/strategies of rhetoric) of language will help you speak specifically about the evidence on the page as it pertains to the global outlook that the piece hints at. The two sets of descriptions set up a much larger concept and that concept permeates into the real world. What is that concept or what are those ideas?
They are wide ranging. The ideas are crucial in the understanding of the piece itself, no doubt, however it (they) reach further? How much further do they reach?
Respond to the prompt in a way that you touch on the larger messages that are inherent in the piece. Do the ideas shape the reality you are currently forming for yourself?
The specific language of the piece are nice examples and activities of rhetoric, so how do they ultimately point to and lead us into answering the bigger question/concept of the piece?
In your quest to develop and support your interpretation as to the message of the piece you will find it important to identify tools of figurative language and how they differ from the literal translations of those same descriptions.
Identify tools of figurative language:
What does that language say? How does it help support your case? Why does the language work?
Furthermore, identify the language that juxtaposes the appearance of the figurative language:
How does that language differ? How does it help support your ideas? What does the language reiterate about the larger picture?
Mark Twain excerpt: (Two views of the Mississippi)
The face of the water, in time, became a wonderful book--a book that was a dead language to the uneducated passenger, but which told its mind to me without reserve, delivering its most cherished secrets as clearly as if it uttered them with a voice.
And it was not a book to be read once and thrown aside, for it had a new story to tell every day.
Throughout the long twelve hundred miles there was never a page that was void of interest, never one that you could leave unread without loss, never one that you would want to skip, thinking you could find higher enjoyment in some other thing. There never was so wonderful a book written by man; never one whose interest was so absorbing, so unflagging, so sparklingly renewed with every reperusal.
The passenger who could not read it was charmed with a peculiar sort of faint dimple on its surface (on the rare occasions when he did not overlook it altogether); but to the pilot that was an italicized passage; indeed, it was more than that, it was a legend of the largest capitals, with a string of shouting exclamation points at the end of it, for it meant that a wreck or a rock was buried there that could tear the life out of the strongest vessel that ever floated.
It is the faintest and simplest expression the water ever makes, and the most hideous to a pilot's eye. In truth, the passenger who could not read this book saw nothing but all manner of pretty pictures in it, painted by the sun and shaded by the clouds, whereas to the trained eye these were not pictures at all, but the grimmest and most dread-earnest of reading matter.
Now when I had mastered the language of this water and has come to know every trifling feature that bordered the great river as familiarly as I knew the letters of the alphabet, I had made a valuable acquisition. But I had lost something, too. I had lost something which could never be restored to me while I lived.
All the grace, the beauty, the poetry, had gone out of the majestic river! I still keep in mind a certain wonderful sunset which I witnessed when steamboating was new to me.
A broad expanse of the river was turned to blood; in the middle distance the red hue brightened into gold, through which a solitary log came floating black and conspicuous; in one place a long, slanting mark lay sparkling upon the water; in another the surface was broken by boiling, tumbling rings, that were as many-tinted as a opal; where the ruddy flush was faintest, was a smooth spot that was covered with graceful circles and radiating lines, ever so delicately traced; the shore on our left was densely wooded, and the somber shadow that fell from this forest was broken in one place by a long, ruffled trail that shone like silver; and high above the forest wall a clean-stemmed dead tree waved a single leafy bough that glowed like a flame in the unobstructed splendor that was flowing from the sun.
There were graceful curves, reflected images, woody heights, soft distances; and over the whole scene, far and near, the dissolving lights drifted steadily, enriching it every passing moment with new marvels of coloring.
I stood like one bewitched. I drank it in, in a speechless rapture. The world was new to me, and I had never seen anything like this at home. But as I have said, a day came when I began to cease from noting the glories and the charms which the moon and the sun and the twilight wrought upon the river's face;
another day came when I ceased altogether to note the. Then, if that sunset scene had been repeated, I should have looked upon it without rapture, and should have commented upon it, inwardly, after this fashion: "This sun means that we are going to have wind tomorrow; that floating log means that the river is rising, small thanks to it; that slanting mark on the water refers to a bluff reef which is going to kill somebody's steamboat one of these nights, if it keeps on stretching out like that; those tumbling 'boils' show a dissolving bar and a changing channel there;
the lines and circles in the slick water over yonder are a warning that that troublesome place is shoaling up dangerously; that silver streak in the shadow of the forest is the 'break' from a new snag, and he has located himself in the very best place he could have found to fish for steamboats; that tall dead tree, with a single living branch, is not going to last long, and then how is a body ever going to get through this blind place at night without the friendly old landmark?"
No, the romance and beauty were all gone from the river. All the value any feature of it had for me now was the amount of usefulness it could furnish toward compassing the safe piloting of a steamboat. Since those days, I have pitied doctors from my heart. What does the lovely flush in a beauty's cheek mean to a doctor but a "break" that ripples above some deadly disease? Are not all her visible charms sown think with what are to him the signs and symbols of hidden decay? Does he ever see her beauty at all, or doesn't he simply view her professionally, and comment upon her unwholesome condition all to himself? And doesn't he sometimes wonder whether he has gained most or lost most by learning his trade?