anorthosite arrowhead region basalt bedrock boundary waters canoe area wilderness canadian shield coldwell complex cumulate rock digital object identifier enger tower extrusive gabbro greenstone belt gunflint range ilmenite intrusion labradorite lake superior lake superior agate laurentian upland lava lopolith mafic magma mesabi range minnesota minnesota department of natural resources misquah hills paleoproterozoic palisade head penokean orogeny quartz rhyolite saint louis river superior national forest syncline thomsonite ultramafic to mafic layered intrusions united states geological survey university of minnesota volcanic group