MODE OF METAMORPHISM OF PEBBLES. 237 



is also the case in the more crystalline parts of the Obermittweida conglom- 

 erate* of Germany ; and it is very marked in the scarcely less noted localities 

 in Norway described by Reuschf. This regularity in the form of the peb- 

 bles and bowlders of these undoubtedly metamorphosed conglomerates is in 

 strong contrast to the conglomerate-like rocks under discussion. In the 

 Black Hills and the other localities mentioned the metamorphism has only 

 gone far enough to produce a finely crystalline schistose matrix, yet in 

 certain cases, the pebbles lack but little of total destruction (p. 215). That 

 the matrix of a fragmental rock could become slowly heated to such a tem- 

 perature, or be subject to such other conditions as are necessary in order that 

 it could crystallize as a coarsely granular granitoid gneiss or granite, and 

 not at the same time destroy the bowlders and pebbles which it contains, 

 seems to me incredible. The explanation of these rocks and of the interlami- 

 nations of granite with slate and schist by metamorphism, implies not only 

 that the fragments and the bands of slate and schist have been able to resist 

 the forces of change during the slow processes which have been sufficient to 

 produce coarsely crystalline material adjacent, but that in situ they have con- 

 tinued to resist these forces during all the time required by the matrices to 

 pass once more into ordinary conditions. The processes embodied in such 

 " selective metamorphism " certainly need explanation. If, upon the other 

 hand, the fragments are regarded as caught in an eruptive rock, and the iu- 

 terlaminations of slate and schist with granite are due to the intrusion of 

 the latter, it only necessitates the capacity of the fragments and layers to 

 resist destruction until the heated material has solidified. As the igneous 

 rock has perhaps been removed from any considerable mass of fused mate- 

 rial, this process would be a comparatively rapid one. Yet under these 

 comparatively favorable circumstances, instances are too well known to need 

 citation of the partial or complete absorption of fragments caught in dikes 

 or other eruptives. If these fragment-like forms are regarded as segrega- 

 tions or partially absorbed fragments in intrusives, their intermingled regu- 

 lar and irregular forms and frequent lack of definite arrangement present 

 no difficulty. 



From the foregoing paragraphs I would by no means be understood as 

 advocating the notion that all the rocks of the fundamental complex are 

 igneous, although in recent years it has been demonstrated that this is the 

 case for many of them. I merely maintain that many clastic rocks which 



(p. 719). These pseudo-conglomerates and the interlamination of granite and slate (p. 562) were re- 

 garded as evidence that the granites and syenites of Vermont are sediments metamorphosed by 

 aqueo-igneous fusion. It was, however, realized that they were thorougely plastic and acted essen- 

 tially like eruptives. It would seem to be necessary in each individual case to show, rather than 

 to assume, that the material of the granites and syenites is actually derived from sedimentaries. To 

 extend the significance of metamorphism so as to cover crystalline rocks which have been in a 

 fluid condition is to make it useless for purposes of discrimination. 



* Erlunterungen zur geologisehen Specialkarte des Konigreichs Sachsen — Section Elterleiu, A. 

 Sauer, p. 31. 



t Die Fossilier fi'ihrenden Krystallinischen Schiefer von Bergen in Norwegen, H. H. Reusch : 

 Deutsche Aufgabe, R. Baldauf, pp. 16, 22-26, 50-56. 



