98 GEOGNOSY AND GEOLOGY. 
guishable with the naked eye. When the mixture is more intimate, the dark 
rock appears sprinkled with white. The porphyritic (dotted) lava is an 
indistinct mixture of uneven earthy fracture, in which lie crystals either of 
leucite or augite, or both. The compact leucitophyre is often very similar to 
basalt. The vesicular, slag-like, spongy, and glassy kinds, are sufficiently 
characterized by their names. The products of atmospheric decomposition 
proceeding from these rocks, are very favorable to vegetation; some 
varieties, as the slaggy, however, resist such influence most energetically. In 
places where this kind exists, as in volcanic or igneous regions, extensive 
tracts of land are barren and desolate. It is almost impossible even for 
cryptogamia to extract any inorganic nutriment from this durable rock. 
Section 3. Basalts. 
This includes several intimate combinations in which augite predominates, 
sometimes, however, replaced by basaltic hornblende. Magnetic oxyde of 
iron is usually associated with the augite, existing sometimes in a separate 
crystalline form. As the mixture of the constituents of basalt is so intimate, 
it is necessary to direct our attention to surfaces which have been acted 
upon by the weather. These plainly indicate the existence of labradorite, 
sometimes replaced by a zeolitic mineral. Olivine (common chrysolite) is 
an extra ingredient, but is so generally found in basait, as to be considered 
an essential constituent by some geologists. 
Basalt. This exhibits various diversities in respect to its state of 
aggregation, these being quite analogous to those which we have considered 
in rocks of undoubted igneous origin, as in leucitophyre ; the names of the 
varieties, compact, earthy, vesicular, slaggy, spongy, and glassy, sufficiently 
indicate their distinguishing characters. This rock is of a very dark color. 
Amygdaloidal Basalt. This is a variety of basalt in which occur 
spherical, ellipsoidal, and irregularly shaped cavities, generally filled or lined 
with crystallized or crystalline minerals, which are mostly zeolitic. 
Section 4. Dolerites. 
This section comes very near to that of the basalts. It embraces - 
mixtures of augite with feldspathic and ferruginous minerals; among the 
latter magnetic oxyde of iron and specular iron are in a more or less 
crystalline condition. 
Dolerite (basaltic greenstone). Augite, generally the predominant 
component, exerts the greatest influence on the character of dolerite. Its 
dark color depends on augite, being varied to lighter by feldspathic 
substances (especially labradorite). Some one or other ferruginous 
mineral appears always to be present. This exerts a great influence on the 
color of the rock; in the .ochrey condition it penetrates the mass, and 
colors it reddish-brown. The components of dolerite are generally 
distinguishable ; there is, however, a modification, exhibiting a less degree 
of crystallization, and forming an insensible transition into basalt; this is 
known as anamesite. Dolerite is exhibited as crystalline, granular, 
porphyritic, vesicular, or slaggy (as in dolerite lava). 
Section 5. Trap Rocks. 
The term trap was formerly made to embrace many species essentially 
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