Complex sentence: Elvan is a name given locally to the quartz-porphyries which occur as dikes in Cornwall; in many of them the matrix contains scales of colorless muscovite or minute needles of blue tourmaline.
Split into simpler sentences: Elvan is a name given locally to the quartz-porphyries that occur as dikes in Cornwall. In many of them the matrix contains scales of colorless muscovite or minute needles of blue tourmaline.