CLASSIFICATION OF IGNEOUS ROCKS 261 



basalt* are relatively poor in silica (46-55%), and most of them in potash and soda, 

 hut rich in Him . magnesia, and iron. Basalts are classed as basic, and some are 

 highly so. Tin- lavas of many basaltic flows were very fluid, and spread out in 

 thin sheets when poured out upon the surface. In cooling, basalt is prone to 

 take- on a columnar st ruci ure (p. 248). The columns of Giant's Causeway and Fin- 

 gal's Cave are familiar examples. 



Basalts graduate insensibly into doleritcs, which may be regarded as basalts of 

 coarse crystalli/ation. Diabase is a rock of similar composition and ophitic texture; 

 that is the pyroxene crystals are separated into thin plates by inter-growths of 

 plagioclaae, 



General names. The difficulty of distinguishing many of the foregoing rocks 

 from each other by any means available in the field, owing to the minuteness of 

 the crystals, and to the gradation of one type of rock into another, makes it desir- 

 able to employ certain general names which will correctly express the leading 

 character of the rock without implying a knowledge of its precise mineral com- 

 position. A convenient term of this kind is greenstone, which merely indicates 

 that the ferro-magnesian minerals are prominent, and give a greenish or greenish 

 black cast to the rock. The greenstones embrace the dolerites and basalts, and 

 some of the gabbros and diorites, and may even extend to the peridotites and per- 

 haps to others. Another convenient name is trap, which may be used for any 

 dark, heavy igneous rock, such as basalt. The term basalt is sometimes used in 

 much the same way. 



Varieties of rock dependent upon conditions. From what has preceded, it is 

 clear that the chemical nature of the liquid magma determines the mineralogical 

 composition of the rock, if it is crystalline; but it may now be pointed out that 

 the same lava which made a plutonic granite, might have made a porphyry, an 

 <>!idian, a pumice, or a tuff, under other conditions of solidification. The same is 

 true of diorites, gabbros, etc. 



A New System of Classification and Nomenclature of Igneous Rocks 

 The current systems of classifying and naming rocks, if indeed they can be 

 called systems, have grown up gradually out of earlier and cruder methods, many 

 of which were inherited from popular usage. Most of the names and definitions 

 came into use before modern methods of study were adopted. These systems, 

 therefore, retain many crudities and inconsistencies, and lack adaptation to present 

 needs and knowledge. A more adaptive and consistent classification is needed, 

 and in response to this need, a new system of classification of igneous rocks has 

 been offered by a group of leading American petrologists. 1 To some extent this 

 proposed system may be extended to metamorphic crystalline rocks. The clari- 

 fication and nomenclature of the sedimentary rocks probably must always remain 

 plastic, to express the various points of view which it is desirable to consider. 



The proposed system includes two parts, a field system and a quantitative 

 system, the one applicable to rocks on casual inspection, and the other only after 

 detailed study. The field system only is here outlined. 



The proposed field system. The proposed field names are based largely on 

 texture and color. Mineral constituents are used for subdivisions when they can 

 be determined easily; otherwise they are neglected. 



1 Cross, Iddings, Pirsson, and Washington. Quantitative Classification of 

 Igneous Rocks See also Johannsen, Jour. Geol. Vol. XIX (1912), p. 317. 



