MUSEUM OF COMPAKATIVE ZOOLOGY. 



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ecncous rock loiii^ after it was formed, and '' the amvirdulos whicli mark 



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this change wore thus slowly developed, and are not the mere fillings of 

 pre-existing cavities." Besides this pseudo-amjgdaloidal structure true 

 amygdaloidal structure was also pointed out. Concerning the sedimen- 

 tary origin of these rocks he remarks : ''But the strongest proof would 

 seem to be in the structure of the so-called scoriaceous amvo:daloi(ls. In 

 those, patches or balls of amygdaloidal material are associated, even sur- 

 rounded by an imperfectly stnttitied material, which is undistinguishable 

 from the true fine-grained sandstones. This association is such that it 

 seems as if it could in no wise be accounted for by metamorphism acting 

 on sedimentary beds, but only by supposing a peculiar mixture of the 

 materials at the time of deposition, which mixture is not such as 

 sediments assume The fact that in sandstones which are in- 

 tercalated between two trap beds the upper parts, for several inches 

 from the hanging wall, are often changed as if by heat, while at the 

 bottom contact there is no such change, cannot be offered as an ob- 

 jection to the metamorphic theory, for it would be in just such regions 

 that metamorphism would naturally occur. But the fact that sandstone 

 material seems to have entered amygdules near the upper part of beds 

 covered by sandstones; that it may fill a well-defined crack extending 

 down into an underlying melaphyr, .... or that melaphyr may nearly 

 surround pebbles, apparently caught up from an underlying conglom- 

 erate . . . . ; these facts, as does the peculiar structure of the scoria- 

 ceous am3'gdaloids above noticed, seem to point to a very dilTerent origin 



for the melaphyrs than a sedimentary one As a whole, then, the 



structural features of these beds remarkably resemble those of true lavas. 

 They have been affected, however, and to a very great extent, by meta- 

 morphism, and this metamorphism has taken place in such a manner, 

 has so heiglitened and carried on the original structure, as It were, that 

 the ordinary proof of their igneous origin, such as contact changes in 

 adjacent sandstones, presence of amygdules, etc., fail, and it seems nat- 

 ural to consider this metamorphism as a vera causa for the whole struc- 

 ture. Certain extraordinary features, however, as noticed above, seem 

 wholly incompatible with this idea, and when considered as true igneous 

 rocks in which great and peculiar metamorphism has taken place, all the 

 phenomena presented seem to be satisfactorily and naturally accounted 



for These changes, however, have been both very many and verv 



great ; so great, in fact, that, as seen above, when once examined they 

 seem almost sufficient to have developed all the pecidiarities of. the beds 

 from sedimentary deposits. The practical importance of the recognition 



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