OF VOLCANIC ROCKS. 27 



are rare when it intersects them and expands over them. Almost invariably it is found 

 accompanying their ranges at some distance, forming itself extensive ranges ; more 

 frequently, however, it occurs in groups or lines of isolated outliers. It may some- 

 times appear to have no connection with the distribution of older volcanic rocks. But 

 closer investigation will always show it to be within or in the neighborhood of the 

 limits of former eruptive activity. The belts along which this had taken place may 

 be locally interrupted for quite a distance, and the gap be filled by some isolated out- 

 breaks of basalt, or the latter may extend the limits of those belts, in length as well 

 as in width. It is a strange phenomenon that these isolated outbreaks of basalt occur 

 particularly in connection with granite. It intersects this rock very frequently in small 

 dykes, and expands over it in thin sheets. This connection of granite and basalt is 

 very conspicuous on the eastern slope of the Sierra Nevada, and along the belt of 

 basaltic eruptions which traverses the middle part of Germany. Such places are often 

 the seat of basaltic volcanoes, which, though now mostly extinct, are also- otherwise of 

 very frequent occurrence. 



Mineral Composition. Basalt is the representative of what Bunsen has called 

 the " normal pyroxenic type," and is thereby, as well as by its specific gravity, mineral 

 composition, and little variety of texture, the reverse of rhyolite, which represents the 

 "normal trachytic type." All the essential ingredients of basalt, and most of the 

 minerals which enter accidentally into its composition, are different from those which 

 are characteristic of rhyolite. Augite, labrador and titanic iron, imbedded in a paste 

 consisting essentially of the same minerals, constitute generally the rocks of this order ; 

 or labrador may be replaced by leucite, nepheline or a zeolitic substance ; or other min- 

 erals may enter into the composition, such as olivine, basaltic hornblende, hauyne, 

 apatite and black mica ; and in many instances basaltic rocks consist merely of a 

 fine-grained aggregation of the different minerals mentioned. Ordinarily, the paste 

 has a microcrystalline texture, and may either contain crystallized minerals imbedded, 

 or may be devoid of them. This mode of texture passes by gradual steps into that of 

 obsidian, which is the only hyaline variety occurring. Among the peculiarities of these 

 rocks, and which may also in some measure be observed with augitic andesite, is the 

 occurrence of rounded cavities filling the rock and giving it a cellu^ir, sometimes 

 spongy appearance. It is different in nature from pumice-stone, which is a modifica- 

 tion of hyaline texture, while those cavities occur in microcrystalline rocks. 



Subdivisions. Fam. 1st. Dolerite (including nepheline-dolerite and the greater 

 part of anamesite). A crystalline aggregation of augite and labrador with titanic iron. 

 Labrador is usually replaced by nepheline, either in part or totally. Accessory min- 

 erals are : olivine, hornblende, apatite, black mica. 



Fam. 2(7. Basalt. Paste of dark-gray or black color, and of the varieties of text- 

 ure mentioned ; it constitutes either the mass of the rock by itself, or encloses olivine, 

 augite and labrador, in crystals or crystalline grains. Besides these are frequently 

 enclosed : titanic iron, black mica, rubellan, zircon, apatite and other minerals. This 

 family comprises nearly the whole bulk of the rocks which constitute the basaltic 

 order, the two other families being of rare occurrence. 



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