The Twin Sisters are famous in the botanical world for their serpentine soils
i.e. soils very high in Mg and low in Ca; they are also high in Ni, Cr, Fe
and Silica. A principal mineral of the Twin Sisters is Olivine. Serpentine
soils and rocks are often mottled like a serpent's skin, thus the name. They
occur at the juncture of continental and oceanic plates as an intrusion of
the earth's crust. They are a worldwide phenomonen.

Few plants are adapted to grow in serpentine soils due to their toxic mineral
content. However the flora is very interesting. Many endemic species and many
unusual forms of plants occur in serpentine. One of the most beautiful wild
flowers in Wa. (some say the most beautiful) occurs in serpentine Lewisia
tweedyi (the common name is not creative, Tweedy's Lewisia).

Another well known serpentine area of Wa. is the Wenatchee Mountains; also
Cypress Island in the San Juans, Burroughs Island and Fidalgo Head. I
understand that the recently discovered Willemi Pine of Australia also grows
on serpentine soils.

Art Kruckeberg, professor emeritus at the U.W. botany department is a world
wide specialist on serpentine geology and botany. For a good, easy, fun read
on the subject pick up "Assembling California' by John McPhee.

This is a brief sketch, but probably more information than you wanted.

cheers,

fred weinmann