For anyone else who was curious about lichens covering “a not insignificant amount” of the earth’s surface, a quick google tells me it’s about 7% (according to e.g. new york times, scientific american, etc)
Edit: oh and estimating the age of an exposed surface by lichen diameter is called lichenometry. I’m seeing stuff about it being used in geological contexts but it makes sense that it could work for old buildings too
It’s always good to support the original publisher and encourage local libraries by reading a hard copy, so I could never endorse piracy, even for people who can’t get their hands on a physical copy. Even though it’s true that both libgen and annas-archive have ebook copies of this particular book (and can easily be found via google), I could never in good conscience direct anyone to such a site.