Mguntain-Masses. 5t 35. Structure of Mountain-masses. In the next kind of ftrudure, that of Mountain. maffes, we have two principal kinds of ftru&ure to d.efcribe. Thefe are the JlratijiedjlruSlure, or that of ftrata ; and the jearned jlruSlure, or that of di- ftind concretions. StratifiedJlruSlure.—When a mountain or mourn, tain mafs, compofed of one fpecies of rock, is di vided by means of parallel feams into maffes whofe length and breadth are greater than their thicknefs, or into what may be denominated Ta bular Maffes, which extend through the whole ( mountain, it is faid to be jlratijied, and the indivi dual maffes are termed jlrata. Of this kind of ftru&ure we have inftances in Granite, Limeftone, Clay-flate, and Mica-flate. Rut if the mountain or mountain mafs, confift of an alternation of dif ferent rocks, as of clay-flate and greenftone, or of gneifs and limeftone, it is faid to be compo fed of beds. The feams that feparate the ftra ta are named jlrata-feams, or feams of ftratifica- tion. Strata are in general front four to fix feet thick in the older formations \ but lefs confiderable in the newer. They alfo vary very much in their pojition. The examination of this appearance of ftrata is of great importance to the geognoft and mineralogical geographer. The