an er it ic ▁( co ar ser ) ▁ground mass ▁are ▁generally ▁referred ▁to ▁as ▁di abase ▁( also ▁called ▁dol er ite ) ▁or , ▁when ▁more ▁co arse - gra ined ▁( c ry st als ▁over ▁ 2   mm ▁across ), ▁as ▁gab bro . ▁ ▁Gab bro ▁is ▁often ▁market ed ▁commer cially ▁as ▁" black ▁gran ite ." ▁ ▁In ▁the ▁H ade an , ▁Ar che an , ▁and ▁early ▁Pro ter oz o ic ▁er as ▁of ▁Earth ' s ▁history , ▁the ▁chem istry ▁of ▁er upt ed ▁mag mas ▁was ▁significantly ▁different ▁from ▁today ' s , ▁due ▁to ▁imm ature ▁cr ust al ▁and ▁ast hen os phere ▁different iation . ▁ ▁These ▁ul tr am af ic ▁vol can ic ▁rocks , ▁with ▁sil ica ▁( Si O 2 ) ▁contents ▁below ▁ 4 5 % ▁are ▁usually ▁class ified ▁as ▁kom ati ites . ▁ ▁Et ym ology ▁ ▁The ▁word ▁" bas alt " ▁is ▁ultimately ▁derived ▁from ▁Late ▁Latin ▁bas alt es , ▁a ▁miss p elling ▁of ▁Latin ▁bas an ites ▁" very ▁hard ▁stone ", ▁which ▁was ▁imported ▁from ▁An cient ▁Greek ▁ β α σ α ν ί τ η ς ▁( bas an ites ), ▁from ▁ β ά σ α ν ος ▁( bas anos , ▁" touch stone ") ▁and ▁perhaps ▁origin ated ▁in ▁Egypt ian ▁b au h un ▁" sl ate ". ▁The ▁modern ▁pet ro log ical ▁term