Two other views are of course possible. If the composition of the globe was originally uniform, the two magmas must have arisen by separation from one of intermediate nature. Hence, in any cauldron in the crust, in place of one of two magmas, an intermediate magma may be presumed to exist, and to split up, from various causes, into a number of parts which are separately erupted at the surface. Charles Darwin's[78] remarks as to the sinking of crystals in a cooling magma, and the consequent production of a trachytic and basaltic type in the same cauldron, led the way to a general acceptance of the theory of _magmatic differentiation_ in laccolites and batholites. W. C. Brögger's[79] brilliant explanation of the variation and succession of types of igneous rock in the Christiania district has had a profound influence on workers in other fields, and has perhaps directed attention away from the parallel possibilities of differentiation by assimilation. The _assimilation theory_ provides the second possible view above referred to. A magma may be modified by the rocks into which it intrudes, so that a "basic" fluid may become charged with silica from a sandstone, the product crystallising as a granite; while an "acid" fluid may become so charged with limestone that diorite ultimately results. A. Harker[80] has discussed both theories clearly, with a strong leaning to the acceptance of magmatic differentiation in the cauldron as the only important cause of variation. R. A. Daly, on the other hand, goes at least as far as Lacroix in France in supporting the theory of assimilation. For him, the primitive igneous magma is already basic, and basalts are therefore the prevalent type of igneous rock. They reach us, moreover, from considerable depths. The acid rocks are formed by amalgamation of this magma with siliceous material lying nearer the earth's surface. Igneous rocks exceptionally rich in alkalies, the so-called "alkaline" series, result from the absorption of limestone in the magma; denser lime-bearing silicates are thus formed, which sink by gravitation, leaving a lighter magma above in which soda has become concentrated. Carbon dioxide liberated from the limestone also plays a part in carrying up the alkalies that might otherwise remain in a lower portion[81]. E. H. L. Schwarz[82] extends Daly's views with an almost romantic fulness. He holds, with Chamberlin, that the primitive globe resulted from the aggregation of basic meteoritic material. The more siliceous crust arose from the withdrawal of magnesium and iron into the depths by long-continued processes of leaching and gravitation. The melting of this crust produces the acid igneous rocks. Igneous cauldrons originate in the heat due to faulting, or to crumpling, or even to the impact of gigantic meteorites. When a molten magma is locally established, variation occurs in it by assimilation of different types of material round it.