The balance of judgment as to differentiation and assimilation, which should be regarded as parallel probabilities rather than as rival propositions, is admirably held by C. Doelter[83], whose chapters on this matter can be appreciated by all geologists. It is of course possible that differentiation of type, from various causes, has already proceeded so far in the earth's crust as to produce noteworthy contrasts in the rocks erupted in different areas. The interior of our globe, on Chamberlin's planetesimal hypothesis, need not have been uniform in constitution, either at the outset or at any subsequent time. J. W. Judd[84] has called attention to the existence of _petrographical provinces_, a conception that has been very fruitful in results. These provinces have been grouped by Harker[85] in two branches, characterised respectively by rocks rich in alkalies and by rocks rich in lime. The former branch appears to be associated with the movements of faulting and block-structure, rather than of crumpling, that have produced E. Suess's "Atlantic" type of coast. The rocks rich in lime, on the other hand, are said to be characteristic of areas that have been folded like the countries bordering the Pacific. The names "Atlantic" and "Pacific" have consequently been given to the two branches, but these terms seem too geographical in their suggestion. Dewey and Flett[86] have put forward a third type of magma, giving rise especially to albite as a primary or secondary constituent, and characterised by the production of pillow-lavas. This type is held to be associated with areas that have steadily subsided, without much folding. G. Steinmann[87], however, has connected the spilites and "ophiolitic" rocks with regions of intense over-folding. So far, there are many cases where it is difficult to assign a petrographic province to one or other of these branches, and the system seems to demand more simplicity within the provinces than nature is prepared to yield. Whatever the causes of variation, it is necessary to mark out by names certain kinds of igneous material, and it is generally accepted that the types thus set up are best based on chemical composition. At the same time, the minerals present in the rock, and also its structure, record certain phases of its history, and deserve an important place in any system of classification. The natural history of an igneous rock is concerned with its mode of occurrence, and no isolated specimen can satisfy the geological investigator. In the field, the porphyritic crystals, which have an important influence on the total chemical composition, may be found to be strangers to the magma, and to have been derived from some mass imperfectly absorbed. The dark flecks and patches in a granitoid rock, so often ascribed, somewhat mysteriously, to local "segregation" in the magma, again and again prove to be metamorphosed and minutely injected fragments of foreign rocks[88].