Talk:Gassmann's equation

The opening statement of this article is taken word for word from Berryman, James G., (1999) "Tutorial: Origin of Gassmann's Equations", Geophysics vol. 64 no. 5. I think it's fine to crib the words, but in that case I also think it's appropriate to cite the article. I don't know how to do it, otherwise I'd do it myself.