Many strikingly banded gneisses are thus of composite origin. Their felspathic granitoid bands can be traced in the field to an igneous source, while their darker and usually micaceous layers can as surely be attributed to the invasion and incorporation of adjacent schists (Fig. 20). But it is quite possible that in other cases the banded gneiss is a sedimentary rock which has undergone what Judd[103] has styled "statical metamorphism." The differences in successive bands are then due to original differences in successive strata; one has yielded a granitic layer, one a layer of quartzite, one, which was more argillaceous, a layer of mica-schist. The bands in such a gneiss record the stratification. [Illustration: Fig. 20. Composite Gneiss formed by intrusion of granite into hornblende-schist. Ängnö, near Saltsjöbaden, Sweden.] Gneisses are often described as if they consisted of layers of various minerals, quartz, felspar, and mica, alternating one with another. As a matter of fact, a gneiss may exist in which there is no differentiation into layers; the whole of the constituents have been drawn out and elongated, any mica present becoming naturally conspicuous by its flattened wisp-like forms. The banded gneisses, on the other hand, where layer-structure is obvious, consist in reality of bands of different rock-types. Sometimes all the layers are granitoid, but one band will contain only quartz and felspar, while another will contain the same minerals with an admixture, and perhaps a great predominance, of mica. G. P. Scrope[104] made an immense step forward when he realised in 1825 that such banded rocks, "the inferior crystalline zones," might be pushed out of position and "protruded" among others "in a solid or nearly solid state." He goes on, "The protrusion of the foliated rocks, gneiss, mica-schist, clay-slate, etc. was chiefly occasioned by their peculiar structure; the parallel plane surfaces of their component crystals, particularly the plates of mica, sliding with facility over one another; while the laminar structure of these rocks was in turn increased during this process, the crystals being elongated in the direction of their motion, as in the case of the clinkstones and pearl-stones of the trachytic formation." After this, there was little left for the later advocates of dynamo metamorphism to put forward. While Darwin[105] recognised how the granite at Cape Town had worked its way insidiously between the layers of a schist, it was left for Michel Lévy to emphasise the part played by what is called _lit-par-lit_ injection in the making of banded gneiss (see p. 120). K. A. Lossen, Johann Lehmann, and other distinguished workers in Germany made clear, on the other hand, the effects of pressure in moulding and reforming crystalline rocks, and even in bringing about the crystallisation of certain minerals in a previously sedimentary mass.