6 RICHTHOFEN — THE NATUR 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 sequel, this indistinctness of external character, as wellas 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. 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- 
1This 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 Uddevalla, 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, 6th Hd., 1866, p. 
601 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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