I can't help thinking that maybe Year of Wonder is just too hard a book to try reading!

i found this with the help of Our Friend Google:

Generally the mineralisation history of these deposits is complex, with successive phases breaking up and re-cementing the existing minerals to produce a brecciated ore body. Locally the fissure fill deposits are known as Rakes and Scrins, with Rakes occurring in large fissures, and Scrins filling smaller cavities and limestone joints.