48 RICHTHOFEN NATURAL SYSTEM 



express the value of the different ingredients entering into their composition. The 

 law of Bunsen, in which are embodied these relations, though true for all eruptive 

 rocks, has never been applicable to those of sedimentary origin, nor to the met- 

 amorphic sedimentary rocks. 2d. The order in which massive eruptions of vol- 

 canic rocks have taken place in different countries, is by no means conformable to 

 a regular succession of gradations in chemical or mineral composition, but shows 

 the existence of several distinct groups, each of which comprises, chemically, a certain 

 portion of the entire range of compounds intermediate between the two extreme types, 

 and has, besides, its own peculiarities of texture and mineral composition. It is for 

 the latter reason that rocks, belonging geologically to two different groups, may be 

 identical as chemical compounds, and yet differ petrographically. We may repeat 

 here that these groups, wherever their mutual relations have been made an object of 

 study, occur in the same order of time, and that the rocks belonging to each of them 

 present generally a great similarity in character wherever they are encountered. 

 3d. The massive eruptions of volcanic rocks are distributed over the globe in certain 

 belts, differing in extent and width as well as in direction. 



There are, among those of a general bearing, two manifest conclusions at which 

 we may arrive by the aid of these facts. The first relates to the origin of the matter 

 ejected. It may be said that equality of physical and chemical properties, and of the 

 mode of occurrence, will commonly involve equality of origin. We have, therefore, 

 to infer, that the source from which volcanic rocks have derived their origin has 

 been similar in nature in every locality, that the definite numerical relations existing 

 in the chemical composition of the matter ejected must exist similarly at that source, 

 and that they must there pervade matter equally in different parts of the globe ; and 

 we may further add, that at the same source the different kinds of matter which cor- 

 respond to the different passages in composition among volcanic rocks, must be ar- 

 ranged, at every locality alike, in a definite order of position in reference to the 

 distance from the center of the earth, since such relation in space can alone explain 

 the definite order of succession in time in which those rocks have been ejected. If 

 we consider the relations existing between volcanic and ancient eruptive rocks, there 

 can be no doubt that all of these have derived their origin from that same source, and 

 that it is of general distribution under the surface of the globe. The second general 

 conclusion relates to the cause and mode of ejection. As like effects imply like 

 causes, and the similarity of the phenomena connected with the massive eruptions of 

 volcanic rocks is conspicuous, we may infer that the agencies to which they have been 

 due, were, in the main, similar in all cases. However accidental and local circum- 

 stances may have caused minor differences, the prominent features in the mode of 

 geological occurrence and geographical distribution cannot be due to them. In how 

 far we may be justified to ascribe the ejection of the granitic and porphyritic rocks to 

 similar agencies, will depend upon the degree of similarity of their part in the struct- 

 ure of the surface of the globe with that of the volcanic rocks. The conclusion must 

 be drawn from those correlations which have already been mentioned, that the funda- 

 mental causes of all eruptive activity are alike, and implied in general planetary pro- 

 cesses which are closely connected with the evolution of the globe. These general 



