842 THE JOURNAL OF GEOLOGY. 



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 igne- 

 ous 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 distinguish- 

 ing peculiarity was characteristic of the parent magma. They 

 are in this sense consanguineous. The presumably homogen- 

 eous parent magma has become heterogeneous by some chemico- 

 physical process or processes, so that different portions of it 

 have different chemical constitutions. The differentiation un- 

 doubtedly takes place according to fixed laws and within limita- 

 tions affected 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 reser- 

 voir, where the process of differentiation is taking place, will draw 

 off magma whose constitution will depend on the phase of dif- 

 ferentiation attained by the parent magma. The phase will nat- 

 urally depend on the time at which the eruption takes place. 



