The rock is phonolite, and is divided into irregular columns.
"A Naturalist's Voyage Round the World" by Charles Darwin
Phonolite is broken shale, a thin, sharp rock that gives forth a pleasant, metallic sound when struck, like shattered crockery.
"The Furnace of Gold" by Philip Verrill Mighels
Trachyte, Domite, and Phonolite.
"Volcanoes: Past and Present" by Edward Hull
CLINKSTONE, called also phonolite, a felspathic rock of the trap family, usually fissile.
"Principles of Geology" by Charles Lyell
It is formed of basalt, trachyte and phonolite, and the soil is very fertile.
"Encyclopaedia Britannica, 11th Edition, Volume 10, Slice 3" by Various
Tertiary intrusive rocks, mainly rhyolite-phonolite dikes, are fairly abundant in the district.
Reference Design Project Book: NUSEL-Homestake