It is interesting therefore to note that G. P. Scrope in 1825 treated of granite, without apology or hesitation, in a work entitled "Considerations on Volcanoes." So far from separating deep-seated from superficial products, Scrope wrote of the molten magma in the crust as "the general subterranean bed of lava." He conceived this fundamental magma, "the original or mother-rock," to be capable of consolidating as ordinary granite. Successive meltings and physical modifications of this granite gave rise, in his view, to all the other igneous rocks. Scrope laid no stress, however, on chemical variations within the magma, but urged that the transitions observable between different types of igneous material established a community of origin. The connexion between lavas and highly crystalline deep-seated rocks, so simply accepted by Scrope, was worked out some fifty years later by J. W. Judd for areas in Hungary and in the Inner Hebrides. The features displayed in thin sections under the microscope were used by Judd, in a series of papers, to substantiate his views; but in France and Germany these features became the source of subtle distinctions between the igneous rocks of Cainozoic and pre-Cainozoic days. The lavas, in which some glassy matter could be traced, were said to be typically post-Cretaceous, and essentially different from those earlier types in which glass was replaced by finely crystalline matter; while the coarsely crystalline igneous rocks were uniformly regarded as pre-Cainozoic. Glassy rocks, such as pitchstone, interbedded contemporaneously in Permian or Devonian strata, were described as "vitreous porphyries," while those known to be of post-Cretaceous date might be styled andesites, trachytes, or rhyolites. Luckily common sense has recently triumphed in this matter, and the relative scarcity of glassy types of igneous rocks in early geological formations has been recognised as due to the readiness with which glass undergoes secondary crystallisation. The discussion has ended by showing that we have no evidence of world-wide changes in the types of material erupted during geological time. At the present day, attention has been focused on the processes that go on in subterranean cauldrons, in the hope of explaining the differences between one type of extruded rock and another. Doctrines of descent have been elaborated, and one of the most subtle systems of classification[59] has been based upon characters that the igneous rock might have possessed, had circumstances not imparted others to it during the process of consolidation. The principle of this classification is, however, obviously correct, if we wish to trace back a rock bearing certain characters at the present day to the molten source from which it came. CHARACTERS OF IGNEOUS ROCKS