Liihological Nomenclature. 239 



raphy, as distinguished from lavas that have arisen from craters 

 and flowed away in radial streams, with the attendant structural 

 distiDctioDs between the two, it will serve a convenient function in 

 the literature of the subject, without being a "cloak of ignorance " 

 in any other sense than lava i?, or many other general, very con- 

 venient and necessary terms. 



There will, doubtless, arise many cases in which it will be im- 

 possible to determine the method of issuance of a given igneous 

 rock, and neither the term lava nor irajp could be used in the 

 restricted sense here proposed, and there may be little funda- 

 mental distinction between the phenomena in the two cases ;. but 

 both the distinction and the terms are serviceable in geological 

 literature, when stripped of the pretentious clothing to which they 

 have no title. 



Prof. Dana has suggested that metamorphic rocks be designated 

 by the prefix meta. If this were generally adopted it would 

 doubtless be serviceable; but the limitations of knowledge being 

 such as they are, it would seem almost necessary to introduce a 

 corresponding prefix to indicate similar rocks of igneous or 

 aqueous origin. For if the simple name, as diorite for example, 

 be understood to imply igneous origin, and the compound term, 

 as meta-diorite, a metamorphoric one, it would be necessary, in the 

 -yery naming of the rock, to assert an opinion as to its origin. But 

 in many cases it is impossible to positively determine the origin 

 of a rock, whose other characteristics may be very well known ; 

 and there would be no convenient term to express this knowledge, 

 without implying knowledge not possessed. In respect to gran- 

 ite, for instance, it is contended, severally, by able geologists, that 

 it may have an igneous, an aqueous, and a metamorphic origin, 

 and yet, in many instances, the working geologist would not feel 

 at liberty to assert that a given granite belonged to either class ; 

 and it would be a sore inconvenience to be obliged to make an 

 implied assertion upon the subject, or else be shut out wholly 

 from the use o£ the term granite. If, therefore, the system of 

 introducing prefixes to designate origin be adopted at all, it 

 should be complete, and yet leave the working geologist at liberty 

 to use the [fundamental term, free from the added signification. 



