THE METEORIC BASALTS. — THEIR STRUCTURE. 205 



is not dependent on age, or on any especial depth of tlje mass at the time of the 

 crystallization ; also that diabase, dolerite, and basalt are not distinct in age, but 

 merely relative terms, indicating coarseness in crystalline texture and extent of alter- 

 ation; for sections of these Greenland basalts could be pronounced, by taking certain 

 portions of them, to be basalt, dolerite, diabase, and possibly gabbro. 



Since this work is published in parts, it has seemed best to place in the 

 first portion, so far as possible, all relating to meteorites, and to end the 

 first part before taking up the terrestrial basalts. 



Owino- to the views of Professor Tschermak, tliat nearly all the mete- 

 orites are tufas, the preceding descriptions are affected by that view, 

 since most of the microscopic study has been done by him. It' appears 

 to the writer that the basaltic meteorites display in general the structure 

 of friable, rapidly crystallized basalts, apparently quickly cooled, and never 

 bound together by the subsequent products of alteration ; few if any of 

 them being fragmental. From this point of view, their general structure 

 in the basaltic variety would be described as divergent, lath-shaped plagio- 

 clase feldspars, lying in a groundmass of pj-roxene (augite, diallage, and 

 enstatite) grains, with some base, feldspar, and iron ores. 



So far as the gabbro type of the meteorites is concerned, the description 

 of the Bishopville form would serve as a general statement of their collective 

 " characters, varied by the predominance of any one of the mineral constitu- 

 ents. The Bishopville form is certainly not fragmental in structure, and it 

 does not seem to the writer that the other meteoric gabbros ai'e so ; hence they 

 may be defined as crystalline-granular masses of feldspar, pyroxene (au- 

 gite, diallage, and enstatite), with various ores of iron, and with or without 

 olivine. In these, however, certain of the constituents may predominate, 

 to the partial or complete exclusion of others. This is no more than the 

 observed variation occurring in different jjortlons of the same terrestrial 

 rock. 



AlthouQ-h mineralogicallv the basaltic meteorites could be divided into 

 many varieties, the same as the peridotites have been, it seems to the 

 writer imnecessary. The terrestrial basalts were divided in the first place 

 chiefly on structural characters and differences in external appearance, 

 and the recently introduced terms, noritc, olivine-gabhro, and oUriiie-norite, 

 appear to be superfluous and unnecessary, although consistent with the 

 common mineralogical nomenclature of rocks, since structurally all can 

 readily be classed under the variety gabbro. 



