conversations with Lyell, etc., I recommend you to describe in a little
detail the nature of the metamorphic schists; especially whether there are
quasi-substrata of different varieties of mica-slate or gneiss, etc.; and
whether you traced such quasi beds into the cleavage slate. I have not the
least doubt of such facts occurring, from what I have seen (and described
at M. Video) of portions of fine chloritic schists being entangled in the
midst of a gneiss district. Have you had any opportunity of tracing a bed
of marble? This, I think, from reasons given at page 166 of my "S.
America," would be very interesting. (539/2. "I have never had an
opportunity of tracing, for any distance, along the line both of strike
and dip, the so-called beds in the metamorphic schists, but I strongly
suspect that they would not be found to extend, with the same character,
very far in the line either of their dip or strike. Hence I am led to
believe that most of the so-called beds are of the nature of complex folia,
and have not been separately deposited. Of course, this view cannot be
extended to THICK masses included in the metamorphic series, which are of
totally different composition from the adjoining schists, and which are
far-extended, as is sometimes the case with quartz and marble; these must
generally be of the nature of true strata" ("Geological Observations," page
166).) A suspicion has sometimes occurred to me (I remember more
especially when tracing the clay-slate at the Cape of Good Hope turning
into true gneiss) that possibly all the metamorphic schists necessarily
once existed as clay-slate, and that the foliation did not arise or take
its direction in the metamorphic schists, but resulted simply from the pre-
existing cleavage. The so-called beds in the metamorphic schists, so
unlike common cleavage laminae, seems the best, or at least one argument
against such a suspicion. Yet I think it is a point deserving your notice.
Have you thought at all over Rogers' Law, as he reiterates it, of cleavage
being parallel to his axes-planes of elevation?