Yeah, I always have to read it a couple of times. I sited this on my blog back in November after I read in a period magazine (after seeing it on another blog). Now I can't find the book.
I seem to remember that they were talking about cottons.
Generally the fashion notes of the time don't mention fiber, because they assume you know what good poplin is made of (silk and wool), just as we now don't say "cotton denim," we just say "denim." The fact that the quotation above is referencing fall and winter clothing gives yet further weight to the idea that this is silk/wool poplin that we're talking about. Here are a few references to back up the idea that fashionable midcentury poplin is not a cotton fabric:
From The Art Journal of London
"While thus the broad fabrics formed entirely of silk ceased to be executed, except in the smallest quantities, the mixed fabric, formed of a peculiar combination of silk and wool, and known as tabinet
still retained its ground in Dublin. For a time even this production of Irish looms was in danger, in consequence of its having been found impossible to form any poplin having a greater width than sixteen inches. This resulted from the excessive brittleness of the worsted yarn, caused by the high degree of finish required, in order to secure for the completed fabric an uniformly smooth and silky surface. Recent improvements in machinery have overcome these, as they have overcome so many other difficulties and obstacles to the advance of manufactures, and now the woollen yarns are made so elastic that poplin can be woven of any width."
From Le Follet
"We have recently seen a very pretty dress of two colours, intended for autumn wear.... A plain long gored skirt of light stone-coloured silk, with highbuttoned body, and very small open sleeves. Over this was worn a tunic of royal blue silk, also gored, reaching down to the knee in front, but longer behind. A low body, pointed at top and bottom, without sleeves, with a postilion basque, completed this very elegant toilette. This style is likely to come into great vogue; but will never be in danger of becoming common, as no lady with any pretension to good taste, would make it in any but rich materials?such as silk, poplin, &c. The tunic and low body made of black silk, would have a very recherche effect, and might he worn with any coloured silk or poplin skirt."
"Frankie and his mother sat looking in the pleasant coal-fire, ... until it was time to go to church. So mamma bid Frankie go up stairs to be dressed, telling Susan to put on his brown poplin, as that was warmest and best suited to the season. ...'Susan, ask mamma if I mayn't wear my gray suit?' Susan went off to intercede, but without avail. There was a raw, cold wind, and it was quite right that he should wear the warmest garments."
As the 19th century progressed there were actually some poplins being made partly or even wholly of cotton (or linen), but they were considered very second-rate and would not have been recommended as stylish in a fashion magazine. From Chambers's Encyclopedia
"PO'PLIN (Fr. papellne). ...the best modern poplins consist of a warp of silk and a weft of worsted, which gives substance, combined with great softness and elasticity, to the material. Cotton, and even flax yarns ore substituted for silk, wholly or partially, in making cheap goods, but they are very far inferior in beauty to the true poplins."
From the 1863 story Husks
"Sarah's hat?a broad-brimmed 'flat' of brown straw?had fallen back upon her shoulders, and the sea-breeze played in her hair, raising the short and loose strands, and giving to the whole a rough, 'frowzy' look. Her plain linen collar and undersleeves showed her complexion and hands to the worst possible advantage. ... Her gray poplin dress had lost most of its original gloss, and being one of Mrs. Hunt's bargains?'a cheap thing, but plenty good for that outlandish Shrewsbury'?already betrayed its cotton warp by creases that would not be smoothed, and an aspect of general limpness?a prophecy of speedy, irremediable shabbiness. Cast loosely about her shoulders was a light shawl, green, with black sprigs?another bargain; and beyond the skirt of her robe appeared the toe and instep of a thick-soled gaiter, very suitable for a tramp through damp sand, yet any thing but becoming to the foot it protected."