Basalts are among the commonest of igneous rocks, and are popularly designated “_trap_,” much used as a road ballast on account of its toughness, which is largely due to its dense texture. The coast of New England is seamed with dikes of basalt, and through the Adirondack and White Mountains there are a host of these dikes. The crests of such mountains, as the Holyoke Range, the Tom Range, the Talcott Mts., East and West Rocks at New Haven, etc., are all basalt sheets. The Palisades, First Wachung and Second Wachung Mountains of New Jersey are sills of basalt. The Lake Superior region is crisscrossed with basalt dikes. That greatest of all lava fields the Columbia Plateau, covering over 200,000 square miles on the Snake and Columbia Rivers in Oregon, Washington and Idaho, is all basalt. So it goes all down through Nevada, New Mexico and California. Porphyry Pl. 55 This is a term which properly refers to texture alone, indicating a lava, which has cooled in such a manner that one mineral has crystallized out of the magma first and developed to a larger size, while the mass of the material formed tiny crystals in which the larger ones are embedded. The large crystals are technically known as _phenocrysts_. The surrounding mass of tiny crystals is termed the _matrix_. This porphyritic structure is especially characteristic of lavas which have been extruded in large masses, and of intruded lavas in such places as sills and laccoliths. The term porphyry today has the above precise meaning. It is a much abused word, and has had all sorts of meanings. In the past it was first used to refer to lavas in general, then it came to be applied to lavas which had been erupted before Tertiary times, that is to all ancient lava sheets. This idea soon proved incorrect, lavas being of the same composition whether ancient or recent. In the West the word is often colloquially used today to designate almost every kind of igneous rock occurring in sheets or dikes, if in any way connected with ore deposits. When the composition of a rock with porphyritic textures can be determined, the name due to the composition is coupled with that due to texture, making such terms as _trachite-porphyry_, _basalt-porphyry_, etc. Tuff