these precambrian rocks are schists and phyllites, often contorted and disturbed. the general line of strike of the formations in the island is from north-east to south-west.} a belt of granitic rocks lies immediately north-west of the central precambrian mass, reaching from llanfaelog near the coast to the vicinity of llanerchymedd. between this granite and the precambrian of holyhead is a narrow tract of ordovician slates and grits with llandovery beds in places; this tract spreads out in the north of the island between dulas bay and carmel point. a small patch of ordovician strata lies on the northern side of beaumaris. in parts, these ordovician rocks are much folded, crushed and metamorphosed, and they are associated with schists and altered volcanic rocks which are probably precambrian. between the eastern and central precambrian masses carboniferous rocks are found. the carboniferous limestone occupies a broad area south of lligwy bay and pentraeth, and sends a narrow spur in a south-westerly direction by llangefni to malltraeth sands. the limestone is underlain on the north-west by a red basement conglomerate and yellow sandstone (sometimes considered to be of old red sandstone age). limestone occurs again on the north coast around llanfihangel and llangoed; and in the south-west round llanidan near the menai strait. puffin island is made of carboniferous limestone. malltraeth marsh is occupied by coal measures, and a small patch of the same formation appears near tal-y-foel ferry on the menai strait. a patch of rhyolitic/felsitic rocks form parys mountain, where copper and iron ochre have been worked. serpentine (mona marble) is found near llanfair-yn-neubwll and upon the opposite shore in holyhead.