I would like to annotate a bit of Sam’s geography.  From Chapter 18 of
Roughing It:
“At eight in the morning we reached the remnant and ruin of what had
been the important military station of “Camp Floyd,” some forty-five or
fifty miles from Salt Lake City. At four P.M. we had doubled our
distance and were ninety or a hundred miles from Salt Lake. “
This would place him at Simpson’s Springs.
“And now we entered upon one of that species of deserts whose
concentrated hideousness shames the diffused and diluted horrors of
Sahara—an “alkali” desert. For sixty- eight miles there was but one
break in it. I do not remember that this was really a break; indeed it
seems to me that it was nothing but a watering depot in the midst of
the stretch of sixty-eight miles.”
Orion identifies this stop as Fish Springs, the dividing point between
what he describes as the “small desert” and the rest of this desert.
“If my memory serves me, there was no well or spring at this place, but
the water was hauled there by mule and ox teams from the further side
of the desert. There was a stage station there. It was forty-five miles
from the beginning of the desert, and twenty-three from the end of it.”
Twain’s memory is, in fact, in error on this.  There was a station,
Dugway, that had water hauled in.  It was about 20 miles from Simpson’s
Springs, where the water was obtained.  It would seem that Twain also
got his directions reversed as his narrative implies the water came
from a more westerly location.  Simpson’s is east of Dugway.
“We plowed and dragged and groped along, the whole live-long night, and
at the end of this uncomfortable twelve hours we finished the forty-
five- mile part of the desert and got to the stage station where the
imported water was.”
Distance wise, this would be the Fish Springs Station, which does have
wells.  There is currently a wild life refuge at this location.  It is
located about 25 miles from Dugway.
“The sun was just rising. It was easy enough to cross a desert in the
night while we were asleep; and it was pleasant to reflect, in the
morning, that we in actual person had encountered an absolute desert
and could always speak knowingly of deserts in presence of the ignorant
thenceforward. And it was pleasant also to reflect that this was not an
obscure, back country desert, but a very celebrated one, the metropolis
itself, as you may say. All this was very well and very comfortable and
satisfactory—but now we were to cross a desert in daylight. This was
fine—novel—romantic—dramatically adventurous—this, indeed, was worth
living for, worth traveling for! We would write home all about it.”
“This enthusiasm, this stern thirst for adventure, wilted under the
sultry August sun and did not last above one hour. One poor little
hour—and then we were ashamed that we had “gushed” so. The poetry was
all in the anticipation—there is none in the reality. Imagine a vast,
waveless ocean stricken dead and turned to ashes; imagine this solemn
waste tufted with ash-dusted sage-bushes; imagine the lifeless silence
and solitude that belong to such a place; imagine a coach, creeping
like a bug through the midst of this shoreless level, and sending up
tumbled volumes of dust as if it were a bug that went by steam; imagine
this aching monotony of toiling and plowing kept up hour after hour,
and the shore still as far away as ever, apparently; imagine team,
driver, coach and passengers so deeply coated with ashes that they are
all one colorless color; imagine ash-drifts roosting above moustaches
and eyebrows like snow accumulations on boughs and bushes. This is the
reality of it.”
This trek through the desert probably ended at Willow Creek
Station.  This would not be the only desert Sam still needed to cross
before arriving in Carson.
Just to provide some perspective on the journey, Richard Francis Burton
(not yet Sir) made the same journey across the desert just the year
before:

Standing upon the edge of the bench, I could see the Tophet in prospect
for us till Carson Valley; a road narrowing in perspective to a point,
spanned its grisly length, awfully long, and the next mail station had
shrunk to a little black knob. All was desert: the bottom could no
longer be called basin or valley: it was a thin fine silt, thirsty dust
in the dry season, and putty like mud in the spring and autumnal rains.
The hair of this unlovely skin was sage and greasewood: it was warted
with sand heaps; in places mottled with bald and horrid patches of salt
soil, while in others minute crystals of salt, glistening like diamond
dust in the sunlight, covered tracts of moist and oozy mud. Before us,
but a little to the right or north, and nearly due west of Camp Floyd,
rose Granite Mountain, a rough and jagged spine or hog's back,
inhabited only by wolves and antelopes, hares and squirrels,
grasshoppers, and occasionally an Indian family. Small sweet springs
are found near its northern and southern points. The tradition of the
country declares it to be rich in gold, which, however, no one dares to
dig. Our road is about to round the southern extremity, wheeling
successively S and SE, then W and NW, then SW and SE, and SW and NW -
in fact, round three quarters of the compass; and for three mortal days
we shall sight its ugly frowning form. A direct passage leads between
it and the corresponding point of the southern hill: we contemplate,
through the gap, a blue ridge where lies Willow Spring Station, the
destination of our party after to morrow; but the straight line which
saves so much distance is closed by bogs for the greater part of the
year, and the size of the wild sage would impede our wagon wheels.
The great desert of Utah Territory extends in length about 300 miles
along the western side of the Great Salt Lake. Its breadth varies: a
little farther south it can not be crossed; the water, even where not
poisonous, being insufficient. The formation is of bottoms like that
described above, bench lands, with the usual parallel and perfectly
horizontal water lines, leaving regular steps, as the sea settled down,
by the gradual upheaval of the land. They mark its former elevation
upon the sides of the many detached ridges trending mostly N and S.
Like the rim of the Basin, these hills are not a single continuous
mountain range which might be flanked, but a series of disconnected
protrusions above the general level of the land. A paying railway
through this country is as likely as a profitable canal through the
Isthmus of Suez: the obstacles must be struck at right angles, with
such assistance as the rough kanyons and the ravines of various levels
afford .
We are now in a country dangerous to stock. It is a kind of central
point, where Pavant, Gosh Yuta (popularly called Gosh Ute) and Panak
(Bannacks) meet. Watches, therefore, were told off for the night. Next
morning, however, it was found that all had stood on guard with
unloaded guns. 
(The City of the Saints, p 454-5)

I suppose Burton was not much of a soothsayer in regards to major
engineering feats, ie railroads and canals.
-- 
 There are more things in heaven and earth, Horatio, than are dreamt of
                          in your philosophy.
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