"4 Classification of rocks and description of physical properties of rock". Engineering Geology Field Manual(PDF)1. US Bureau of Reclamation, Technical Service Center Engineering Geology Group. 1998. pp. 57–90. Arquivado dende o orixinal(PDF) o 17 de xuño de 2015.
"4 Classification of rocks and description of physical properties of rock". Engineering Geology Field Manual(PDF)1. US Bureau of Reclamation, Technical Service Center Engineering Geology Group. 1998. pp. 57–90. Arquivado dende o orixinal(PDF) o 17 de xuño de 2015.