MUSEUM OF COMPAUATIVE ZOOLOGY. 101 



geneous rock long after it was formed, and " the aniygdules which mark 

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

 pre-existing cavities." Besides this pseudo-amygdaloidal 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 amygdaloids. In 

 these, patches or balls of amygdaloidal material are associated, oven sur- 

 rounded by an imperfectly stratified 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 tlie 

 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 sever.il 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 tlie metamorphic theory, for it would be in just such regions 

 that metamorphism would natui*ally occur. But the fact that sandstone 

 material seems to have entered amygdnles 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 amygdaloids above noticed, seem to point to a very different 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 heightened 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 catcsa for the whole strnc- 

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

 wliolly 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 ver}- manv and vorv 



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

 seem almost sufficient to have developed all the peculiarities of tlie beds 

 from sedimentary deposits. The practical importance of the recognition 



