What butterflies and slate have in common is that both started out as something else entirely...

Until the movement of the earth's tectonic plates, slates were just shales or humble mudstones. The enormous pressures involved in this shift then changed their identities forever. These titanic forces realigned the minerals, mica and chlorite, contained within the deposits, forming smooth, flat layers, until they become new rocks entirely - metamorphic rocks, from the Greek, 'to change', hence the butterfly connection.