Good information, all; thank you.
Interesting to think of cotton as extreme cold wx gear.
I guess I've been inculcated in the "cotton kills" mentality to the degree that it shocks the system a bit to think of cotton in the cold.
Regarding poly/cotton, the US Army abandoned poly/cotton field uniforms in the early eighties and went to "nyco" (nylon/cotton) instead. The old OG 501 poly/cotton field uniforms tore easily, weren't warm in the winter, and didn't breath well in the summer. The nyco BDU's (now being replaced by nyco ACU's) were FAR tougher, were warmer in cold wx, and breathed a bit better. Speaking of tough, I have a pair of BDU pants issued in 1982 that have seen a lot of hard use (military, work projects, hiking, camping, etc.) that are still more or less wearable. (My wife doesn't regard them as particularly wearable. I regard them as merely "well broken in.")
The problem I've had with nyco BDU's is that they do NOT dry quickly when compared to 100% nylon pants. They're great pants in fair wx, but I don't wear 'em if there's going to be snow on the ground, rain in the forecast, or a lot of other wet scenarios like stream crossings and meadow slogs. Here in California, in the mountains even if the night is -15C, the day may be +5C (or more). Cotton, poly/cotton, and nyco generally aren't good options in snow country here.
I've found that 100% nylon lightweight hiking pants are lighter, dry
very quickly, and are pretty tough (not nearly as tough as BDU's, but pretty tough). The 100% nylon pants were ideal for my trip to humid Japan last summer for both hiking and general travel wear. Any cotton items I wore didn't dry out overnight, by my nylon pants dried quickly and well. For "heavy" XC travel (a lot of brush, thorns, etc.), the 100% nylon pants are too light; they snag, sometimes tear, and don't offer much penetration protection against thorns. Of course in really heavy thorns, BDU's aren't all that much protection either, but they're far better than 100% nylon hiking pants.
Typical 100% nylon hiking pants are a bit light for cold wx, but layering long johns underneath helps for mildly cold wx. Given the wide temperature swings here, layering is key. In cold wx, I'll carry lightweight long johns, heavy weight long johns, nylon pants, full zip fleece pants, and full zip GoreTex pants, which I can layer/adjust as needed. For the upper portion of my body, I'll do something fairly similar: wicking T shirt, wicking turtleneck, fleece shirt, sweater, fleece vest, fleece jacket (or down if extremely cold), and GoreTex Jacket. I also layer for my head: lightweight fleece hat, heavyweight fleece hat, and fleece-lined GoreTex hat. I also have a heavy fleece hood and fleece face mask for truly cold wx. I also have liner gloves, fleece convertible mitten/gloves, and GoreTex overmitts with fleece liners.