THE ORIGIN AND DESCENT OF ROCKS. 417 



Closely allied to the peridotites are rocks which are made up largely 

 of a single basic mineral, as augitite, pyroxenite, hornhlendite, rocks 

 essentially formed of the minerals augite, pyroxene, and hornblende 

 respectiveh\ It will be noted that in these rocks the magma became 

 quite simple in nature, just as at the acid end of the series certain rocks 

 become comparatively simple from the concentration of the acid element, 

 as in certain acidic granites, felsites, etc. (See pp. 523-524.) 



The basalts. — The term basalt is used in a somewhat comprehensive 

 way to embrace dark, compact, igneous rocks that appear to be nearly 

 homogeneous, owing to the minuteness of the crystals, which are usually 

 so small as to be identifiable only under the microscope. In some cases 

 the crystals are scattered throughout a ground-mass after the por- 

 phyritic fashion. In some of these cases there is a true glassy base, and 

 in such cases the rock does not strictly belong in the holocrystalhne 

 group. In the more typical cases the constituent minerals are ver}^ 

 minutely crystallized and intimately intermixed. The leading minerals 

 are plagioclase (usually labradorite or anorthite) and pyroxene (usually 

 augite), with olivine and magnetite or ilmenite usually present. There 

 is a considerable range in chemical nature, but the basalts are relatively 

 poor in sihca, usually also low in potash and soda, but rich in hme, mag- 

 nesia, and the iron oxides. They are classed as basic and are sometimes 

 highly so. The magmas of the basalts are especially fluid, and when 

 poured forth upon the surface easily spread out in thin sheets. In cool- 

 ing they are prone to take on a columnar or basaltic structure, the 

 columns standing at right angles to the surfaces exposed to coohng. 

 The columns are sometimes curved, owing to the pecuhar attitude of 

 the coohng surface. The columns of Giant's Causeway and FingaFs 

 Cave are familiar examples. 



The dolerites. — The basalts graduate insensibly into the dolerites; 

 indeed the dolerites may be regarded simply as basalts of coarser crys- 

 talhzation. The minerals are evident to the eye and range up to medium 

 size. The more abundant minerals are plagioclase feldspar (labradorite 

 or anorthite), T\ith one or more of the ferromagnesian minerals (augite, 

 oh vine, or biotite), and magnetite or ilmenite. In the growth of the 

 minerals one crystal frequently embraces others, giving an ophitic struc- 

 ture. The dolerites have many varieties, due either to accessory minerals 

 or to the development of some of the constituents more amply than the 

 rest. The type may be said to consist of plagioclase and augite, the 



