I promise you this: no Goretex is involved.
Here it is, survival fashion for that trip to Leadville, straight from a seasoned traveler (quoted from the Colorado Miner and appearing in the Denver Daily Tribune, February 4, 1879):
Zounds! Sounds like the properly dressed fellow wears nearly all he owns, just to make the trip. And not a word said about the properly dressed lady. One can only imagine.
Wear a fur cap; carry a scarf that will fold around your head and neck with about six thicknesses; put on a thick flannel shirt and two more over that; one pair of drawers, and two more other that; let your vest, pants and coat be heavy, loose boots and a pair of overshoes may keep your feet warm; put a bottle of the best spirits in your pocket, for arctic emergencies, and then envelop yourself in a pair of California blankets as soon as you get on the coach.
Now, here's an interesting thing: I wondered about these California blankets. Just previous to this quote, there's the line "... we advise everybody to go warmly clad, and to carry a pair of heavy blankets along..." Hence, "California blankets" must be very warm. But look up the term in a slang dictionary (I used the Random House Historical Dictionary of American Slang), and "California blankets" is hobo slang for "sheets of newspaper used for bedding or warmth" circa 1926. Apparently the term went from one extreme (really really toasty warm) to the other (barely adequate covering on a warm California night) in the almost 50 years between 1879 and 1926.
Next up ... more about the 1879 "Leadville craze."