Fresh wolf tracks were plenty all along the bank of the stream; panthers and bears abounded in that section, and the wilderness beyond me was never explored, and hardly penetrable, so dense was the undergrowth of dwarf firs and swamp cedars.
"The Autobiography of a Journalist, Volume I"
William James Stillman
The lady seemed not to be made of penetrable stuff.
"Tales & Novels, Vol. IX [Contents: Harrington; Thoughts on Bores; Ormond]"
Maria Edgeworth
At present, it is difficult to believe that the rays of the sun are penetrable by each other, but who knows what may happen to prove it?
"A Philosophical Dictionary, Volume 5 (of 10) From "The Works of Voltaire - A Contemporary Version""
François-Marie Arouet (AKA Voltaire) Commentator: John Morley Tobias Smollett H.G. Leigh