Page:The Journal of geology (IA journalofgeology11893univers).pdf/426
of different rock magmas to form an intermediate modification are exceedingly local, and appear to be confined to narrow limits along the junction of one body of rock with another.
The genetic relationship between the various kinds of igneous rocks belonging to a center of volcanic activity, which is plainly indicated by their chemical, mineralogical and geological relationships, is in the nature of a generic connection. They have originated from some common magma or parent stock, and to a very large extent are characterized by whatever distinguishing peculiarity was characteristic of the parent magma. They are in this sense consanguineous. The presumably homogeneous parent magma has become heterogeneous by some chemico-physical process or processes, so that different portions of it have different chemical constitutions. The differentiation undoubtedly takes place according to fixed laws and within limitations afffected by the original constitution of the magma, and by the external controlling conditions or agencies. Further than this we shall not venture in the present article. It will be sufficient to consider some of the consequences of the general principles of magmatic differentiation.
First. If differentiation is controlled by external agencies or conditions, such as changes of temperature and pressure, which depend largely on the environment of the magma, then the results of differentiation should vary when the external conditions vary. It is not to be expected, therefore, that similar magmas will always yield the same results when differentiated, within certain limits. They may have experienced quite different physical conditions. The more uniform the conditions the more concordant the results.
Second. Since the process of differentiation requires time, is progressive, and, from geological evidence already alluded to, often continues for ages, it follows that eruptions from a reservoir, where the process of differentiation is taking place, will draw off magma whose constitution will depend on the phase of differentiation attained by the parent magma. The phase will naturally depend on the time at which the eruption takes place.