30 GEOLOGY 



The Peridotites. These stand at the basic end of the series, having 

 been formed from a magma in which the silica was low (39-45%), as were also 

 the alumina, lime, and the alkalies, but in which the magnesia was high 

 (35-48%). The rock consists very largely of the mineral olivine (a mag- 

 nesium-iron silicate), associated with pyroxene, magnetite, and other very 

 basic minerals. Little or no feldspar is present. The peridotites are much 

 less abundant than the preceding classes. 



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

 a single basic mineral, as augite, pyroxenite, hornblendite, rocks essentially 

 formed of the minerals augite, pyroxene, and hornblende, respectively. 



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

 for dark, compact, igneous rocks that appear to be nearly homogeneous 

 owing to the smallness of the crystals which are usually so minute as to be 

 identifiable only under the microscope. 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 silica, and usually 

 in potash and soda, but rich in lime, magnesia, and iron. The basalts are 

 classed as basic, and are sometimes highly so. The lavas of basaltic flows 

 were especially fluid, and spread out in thin sheets when poured out upon 

 the surface. In cooling, basalt is prone to take on a columnar structure 

 (p. 17). The columns of Giant's Causeway and Fingal's Cave are familiar 

 examples. 



The dolerites. The basalts graduate insensibly into the dolerites; in- 

 deed the dolerites may be regarded as basalts of coarser crystallization. The 

 minerals are evident to the eye and range up to medium size. In the growth 

 of the minerals, one crystal frequently encloses others. The dolerites have 

 many varieties, due either to accessory minerals, or to the development of 

 some of the constituents more amply than the others. The type may be said 

 to consist of plagioclase and augite, the other minerals being regarded as 

 accessories. The varieties are usually designated by prefixes, as olivine- 

 dolerite, etc., but special names are also used for some varieties. 



The ancient dolerites have usually undergone internal changes and such 

 rocks are often called diabases. While the use of the term has not been 

 uniform, it accords with the better practice to regard the diabases simply as 

 partially altered dolerites and basalts. 



General names. The difficulty of distinguishing many of the foregoing 

 rocks from each other by any means available in the field, owing to the minute- 

 ness of the crystals, and to the gradation of one type of rock into another, 

 makes it desirable to employ certain general names which will correctly 

 press the leading character of the rock without implying a knowledge of the 

 precise mineral composition. A convenient term of this kind is greenstone. 

 which merely indicates that the ferro-magnesian minerals are prominent 

 and usually give a greenish or greenish-black cast to the rock. The green- 



