BASIC VOLCANIUS OP HEMLOCK FOEMATION. 97 



objection to it can not be given better than by quoting the words which 

 Zirkel uses in the discussion of the metamorphism of rocks : ^ 



Bei solchen metamorphisch veraiiderten Gesteinen ist es nicht zweckmassig, sie 

 mit dem Nameu desjenigen Typus zu belegen, dem sie durcli die Veriinderung ahnlich, 

 oft bios scheiubar ahulicb gewordeu siud. Eiue solcbe Bezeichuuug werde uur zu 

 missverstandlicbeQ Aufl'assuug der vou dem Gesteiti gespieltea geologischeu Eolle 

 fiibren, welche niemals ausser Acht gelasseu warden darf. Uud so ist es deiin ent- 

 schieden vorzuzieben, der Beneimnng solcber Gesteine eine Form zu geben, in whelcher 

 zuvorderst auch zum Ausdruck kommt, was sie friiher geweseu sind, und nicht einen 

 Namen zu wiiblen, der sie in erster Linie zu etwas stempelt, mit welchem sie genetisch 

 keine Gemeinscbaft baben. 



Using these terms in the way suggested by Cross, attention is most 

 pointedly directed to that variety of rock which the secondary product now 

 resembles mineralogically, rather than to the type from which it was derived, 

 and which in all likelihood it still resembles most closely in its chemical 

 constitution. Whether or not a petrographer will use the term "metadio- 

 rite" or the term " metadolerite " (diabase) for a metamorphosed dolerite 

 will depend on whether or not he prefers to emphasize the present miner- 

 alogical composition of the rock, or its original characters, and thereby its 

 chemical constitution and genetical relations. In the present report the 

 terms "metabasalt" and "metadolerite" are used as including all those 

 altered rocks which demonstrably were originally basalts and dolerites. 



These same strictures hold good in the case of Griimbel's term "epidior- 

 ite," when used, as it is very commonly, in the literature of the Lake 

 Superior region and elsewhere, for rocks avowedly derived from dolerites 

 (diabases), and characterized by the presence of fibrous secondary amphi- 

 bole. It is preferred, in accordance with the above statement, to use the 

 term "epidolerite" (epidiabase) instead of "epidiorite" for such altered 

 dolerites. None of these rocks, unless extremely changed, would resemble 

 chemically a diorite, and we have come of late years to rely more and more 

 upon the chemical composition, combined of course with the mineralogical 

 composition and textures of the rocks, to separate the various kinds from 

 one another. As stated above, the term "epi," associated with the rock 

 name, has come more and more to be restricted in its use solely to a rock, 

 the epidiorite, characterized by a specific alteration product, the amphibole. 

 In respect to this restriction to specific alteration, the term corresponds to 



' Lehrbuch der Petrographie, F. Zirkel: 2d ed., Vol. I, p. 573. 

 MON XXXVI 7 



