53 INTERNAL STRUCTURE. hill. The columns are fometimes jointed, fo that the convex extremity of the one column is' fitted to the concave extremity of the other ; and thefe columns are ufually compofed of globular diftindt concretions. Thefe globular concretions are com pofed of curved lamellar concretions. The fpaces between the different globular concretions are compofed of a loofer-matter than the concretions themfelves; and it is by the falling out of this lefs compadt fubftance that the ftrudture of fuch colymns is firft developed. No rock thews this kind of ftrudture more dif- tinctly than Bafalt: in it we have all the varie ties of the feamed ftrudture, from the fmalleft, which is the lamellar diftindt concretion, to the largeft, which is formed by the grouping of co lumns. This kind of ftrudture occurs alfo in Porphyry and Greenftone. Lava never prefents any of the varieties of the feamed ftrudture ; a negative cha- radter which fufliciently diftinguifhes it from greenftone or bafalt, with which it has been con founded. Another kind of feamed ftrudture which deferves . to be deferibed is the tabular feamed JlruSlure. It is diftinguifhed from the lamellar by being al ways ftraight and much thicker. It is'generally from three to nine feet in length, and rarely thick er than two or three inches. Bafalt, in the lower parts of an individual depofition, has often this kind of ftrudture. At firft fight it is not unlike ftratification.