70 RICHTHOFEN NATURAL SYSTEM 



form " trap rocks " and "lava." One common cause is thus assigned to both meta- 

 morphic and eruptive (including volcanic) action, the latter being considered an ad- 

 vanced stage and ultimate result of the former. The immense action of metamorphism 

 is an undeniable fact. But while formerly, on account of the violent agencies which 

 were- supposed to have been required for it, and the growing conviction that heat alone 

 could not produce effects on such an enormous scale as had been suggested, the sub- 

 ject had to be treated with caution, and any extreme assumption was received with 

 doubt, if not with a certain repulsion, the condition of things, in this respect, has of 

 late undergone a great change, as scientific experiments, and especially those of Dau- 

 bree, have demonstrated the extent of the influence of water and pressure in producing 

 metamorphic action. They have proved the remarkable fact that, at a comparatively 

 low temperature, and with the aid of pressure, the effects of water, if continued for a 

 sufficient time, particularly when it is charged with alkaline substances, will be able 

 to produce changes in the nature of rocks which surpass even the most audacious 

 assumptions of former time. An apparently safe foundation was now given to the 

 widest generalizations, and the consequence is, that an almost unlimited action is at the 

 present time brought to the account of metamorphism. 23 The least founded concep- 

 tion, however, of the faculties ascribed to it, we consider to be its supposed sole in- 

 strumentality in the production of volcanoes and the eruptive action of former ages 

 from portions of the shell of sedimentary rocks. It is true that a great additional 

 degree of apparent probability has been given to the metamorphic theory of eruptive 

 rocks by the results of the microscopic examination of rocks so successfully instituted 

 by Sorby, since they prove a remarkable similarity, in the minutest texture, of the 

 minerals constituting granite and some cognate non-foliated rocks, with the main in- 

 gredients of certain foliated rocks made up of silicates, such as gneiss and micaschist. 

 These had been observed long before to form the last link of a series which may be 

 traced, by slow gradations, from unmetamorphosed sedimentary rocks, through those 

 which are undoubtedly metamorphosed, up to the crystalline foliated rocks mentioned. 

 The newly discovered facts appeared to indicate that granitic texture is the most ad- 

 vanced stage in a progressive series of molecular changes, and, carrying the argumenta- 

 tion still farther, the assumption was apparently justified that a corresponding origin 

 must be ascribed to other rocks, such as those of volcanic origin, which are connected 

 with granite by another long and gradual line of passage. 



In drawing up our argument against this theory, (including all those doctrines 



23 It may be frequently noticed, that those eminent men who, by creating a firm basis for induction, have pointed 

 out the path through regions in the field of science the knowledge of which had consisted before of a confused accumula- 

 tion of facts and suggestions, did themselves apply the newly acquired views within moderate limits, while sweeping general- 

 izations on the same basis were usually made by others. It is so in the present case. The limits within which Daubree 

 himself has applied his ingenious conclusions from his experiments on the action of superheated water and pressure upon 

 silicates, will probably never lie drawn any closer. No discovery, however, could have been more opportune to those who 

 had advocated before the origin of eruptive by the remelting of sedimentary rocks. Against the form of this theory which 

 was first proposed by Ilutton, and enlarged by Lyell, Babbage, Herschel and others, similar objections could be raised as 

 against that form of the theories of the first class which was held by Bach. With the aid of the new views acquired, 

 however, it was remodeled and brought into its present shape. A far greater extent is thus being given to the conclusions 

 from Mr. Daubree's experiments than their author ever intended. 



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