E-NDUCH.J ERUPTED ROCKS INTRODUCTION. 201 



begin, is frequently almost impossible to determine. Taking the view 

 which presents metamorphic rocks as the resultants of the action of heat 

 upon originally sedimeutiary deposits, an action often accompanied by 

 disturbances of position, the detinite line between them and truly erupt- 

 ive rocks cannot so readily be recognized. As a rule, it may be stated 

 that all those rocks are to be considered as eruptive which break through 

 or are ejected through fissures in rigid strata or masses, and either reach 

 the surface or remain hidden beneath it. The latter case includes the 

 phenomenon of intrusion, which, however, occurs more particularly ia 

 sedimentary formations. Within them the difference of regularly de- 

 posited and eruptive material is by far more evident than in the former 

 instance. Although the strata of sedimentary beds may have assumed 

 any position, any angle of inclination, the intrusion or ejection of erupted 

 rock is always sufficiently characteristic to be recognized, both by the 

 different constitution of the rock and by the result of the forms produced. 



Here, too, as in the preceding case, the erupted rocks may reach the 

 surface or may only reach within a certain distance of it. Erosion, in 

 the latter instance, not unfrequently discloses them. If they have 

 reached beyond the exterior limits of the sedimentary beds, their pres- 

 ence is generally indicated by the formation of isolated hills, mount- 

 ains, mountain groups, or plateaus. Thus a ready recognition, sepa- 

 rating them from their surroundings, is established. 



It lies in the nature of the subject that we should find almost innu- 

 merable varieties representing this class of rocks, and investigations have 

 thus far supplied us with an ample list of names. In mode of occurrence, 

 in age, and in lithological character the erupted rocks differ greatly among 

 themselves, and to these features is due the large number of specific dis- 

 tinctions that have been made. As a rule, it may be stated that the 

 older eruptives differ more from the metamorphics in their chemical na- 

 ture than some of the younger ones. A metamorphic series is merely the 

 altered representative of some distinct group of beds, while an eruptive 

 one, although it may be but the same group, is so thoroughly changed by 

 fusion and subsequent cooling, that every vestige of its former physical 

 condition is obliterated. In metamorphic groups stratification may in 

 I)art be preserved, the differences indicating origin from mineralogically 

 separable beds may be noticeable, but an erupted rock is usually so 

 thoroughly and uniformly remodelled, that no clue to its original condi- 

 tion can be obtained from a study of its physical character only. It is 

 impossible to draw any line of distinction which would divide the entire 

 series of eruptives into appropriate, well-defined groups. Bun sen has 

 attempted a primary classification (to be alluded to below), but it is 

 certainly insufficient. At best, they can be said to belong to one grand 

 division of the geological history of our earth, supplying one important 

 factor in the formation of its crust, and fulfilling a detinite mission as 

 regards its exterior surface. For systematic purposes it is requisite, 

 however, to adopt some basis of separation, even if artificial, and for that 

 l)urpose we can scarcely find one answering as well as the chemical. 



Without drawing the lines too closely, a chemical primary classifi- 

 cation has a more or less direct bearing upon one very important ques- 

 tion, viz, that touching the genesis of the erupted masses. Although 

 by no means sufficiently indicative of any one method of operation, the 

 results as furnished us by chemical examination afford indirect hints 

 at least as to the character of the agents employed in the generation of 

 eruptives. So far as our observations extend, their present existence 

 is attributable to one main cause. This cause is heat. Kot only do we 

 infer this from the demonstrations accompanying eruptions of the pres- 



