Roman sails: leather, linen, or?

[richsc]

An author who will be writing a book on the technology of textiles throughout history asked me what Roman sails were made of. Searching threads from a few years back I read possibly leather, and possibly linen. Has there been any new thinking on this or are those the two likely ones? 

I also read that 'canvas' is just a term for the weight and weave of a fabric like linen, so does not pertain just to cotton.