Structure of Formations. 59 ? ' ft ratification. It alfo occurs in columnar por phyry. The laft or third kind of feamed ftruaure, is the , t large globular or maflive, in which all the dimen- fions are nearly alike. It occurs alone, (that is, without any other kind of ftruaure), and is from one to three or more fathoms in diameter. The larger balls Ihew lamellar diftina concretions, which, we may obferve, are .always more folid the nearer we approach the centre. The roundilh balls of granite found difperfed over low countries, have been confidered as bowl der or rolled ftones, and many theories have been formed to account for their tranfportation. The granite of the illand of Arran prefents this kind of ftruaure. Structure of Formations. 37. By a Rock-formation we underftand a deter minate affemblage of fimilar or diffimilar rock- maffes, which are charaaerifed by external and internal relations as an independent whole, that is, as an unity in the feries of rock formations. Thefe maffeS are either Jimple or compound. When the mafs is uniform throughout, as is the cafe with Ifimeftone, or Sandftone, it conftitutes what may be denominated a Jimple formation. Granite, r npifs and Mica-flate, are alfo of this kind. When diffimilar