6 RICIITHOFEN — THE NATDRA SYSTEM 



among these is the entire absence of what we could call " genus," or "species," not to 

 mention " individual." If we should succeed in discovering some natural group to 

 which we might apply the term "family,'-' (though even this can never be used in 

 petrology in as definite a sense as it is in the organic kingdoms) we should find 

 it made up of an infinite number of varieties ; and if we should be able to 

 establish several groups, the main types of which are conspicuously distinct in 

 nature, we should find them linked together by gradual passage in chemical and min- 

 eral composition. It appears indeed utterly impossible to draw distinct boundaries 

 between cognate groups. Certain names, such as those of granite, syenite, quartzose 

 porphyry, trachyte, basalt and others, have been applied to designate distinct types of 

 crystalline rocks, which can easily be recognized wherever met with. But, practically, 

 they have to be used for larger groups of rocks, in which those distinct types appear 

 like luminous centers, surrounded by clouds of varieties blending with each other in 

 such a way as often to render it arbitrary whether to bring a certain rock within one 

 or the other denomination. But there are other groups of rocks, of larger dimensions 

 than the former, the nomenclature of which is far more indistinct, and which, in regard 

 to classification, may indeed be said to be still in an entirely nebulous state. The 

 vague and arbitrary mode in which different names are used for them shows plainly 

 that difficulties in regard to them are greater than with other rocks. As will be seen 

 in the secpael, this indistinctness of external character, as well as of designation, applies 

 particularly (with the exception of basalt) to those rocks in the composition of which 

 silica takes a less prominent part, while those which are richer in silica offer much 

 more distinct characters. Most conspicuous among the names applied for the former 

 are those of "trap," "greenstone," and "porphyry." The latter two may be con- 

 veniently used to designate groups of rocks having certain external characters in com- 

 mon, but as generic terms they should all be completely abolished. 1 They never con- 

 vey a definite conception, as each of them is used for a great variety of rocks, and 

 they have only too often been made to serve as a convenient cloak to cover ignorance. 

 The perception of these difficulties has caused the idea to be almost universally 

 accepted, that only an artificial system of eruptive rocks can be established ; that is, 

 that classification should be made dependent on one certain principle previously as- 



1 This applies chiefly to the term" Trap," (or trapp) which had originally a definite meaning, but has gradually been 

 extended with wonderful elasticity. It was first introduced by Torbern Bergmann for a very ancient, dark colored, augitic 

 rock of Uddcvalla, in Sweden, which is arranged in superposed layers abutting against the slope of the hill in the shape of a 

 stairway (trappar in Swedish). This rock would be called "diabase" in modern nomenclature. The name "trapp" was 

 then applied to other, and gradually to all, dark colored eruptive rocks, particularly to such as were found occurring in 

 dykes ; afterwards the " greenstones" (which name, too, has shown itself capable of wonderful distension) were included in 

 that denomination ; and finally it has become still more comprehensive, though not quite to the same extent with every au- 

 thor. Its present meaning may best be seen from the following passage by Lyell, (Elements of Geology. Cth Ed., I860, p. 

 G01 of Am. Ed.): " This term (lava) belongs more properly to that (melted matter) which has flowed either in the open air 

 or on the bed of a lake or sea. If the same fluid has not reached the surface, but has been merely injected into fissures 

 below ground, it is called trap." Thus, the same name which was originally applied to a distinct rock of ancient origin has 

 been generalized so as to express now a mode of occurrence; and, what is more remarkable, a mode of occurrence the very 

 reverse of that exhibited by the original type, which must be supposed to have been flowing on the bed of the sea, and the 

 position of which in relation to neighboring rocks can never be explained by assuming it to have been" injected into fissures 

 below ground." Would it not be better to drop such a name altogether ? 



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