Indeed, this 43 deficiency of soda in sedimentary rocks is so characteristic of secondary rocks that it may with some safety be applied to discriminate between the two classes of substances in cases where Petrological distinctions of other kinds break down.
"The Birth-Time of the World and Other Scientific Essays"
J. (John) Joly
Of course in these days, since the application of the microscope to the study of rocks in thin sections, Darwin's nomenclature and descriptions of the Petrological characters of the lavas appear to us somewhat crude.
"Darwin and Modern Science"
A.C. Seward and Others