L. V. Pirsson — Classification of Igneous Rocks. 279 



a text book also the classification should be formed in 

 such a manner as not to cut the student off from the his- 

 torical aspect of the subject, or from current usages, and 

 the volume of literature that has grown up based upon 

 them. For these reasons a system has been adopted that 

 is founded upon the older qualitative one, with modifica- 

 tions tending to greater definiteness in the statement of 

 mineral composition, and at the same time a quantitative 

 element has been introduced, and carried as far as seems 

 advisable at present, in order not to introduce too great 

 complexity into the scheme, and too great a departure 

 from prevailing usages. The quantitative elements are 

 derived in some degree from the quantitative chemical- 

 mineralogical classification previously mentioned and 

 the scheme is somewhat similar to that advanced by 

 Iddings,^^ but with important modifications. It is shown 

 in Table No. 3. 



Explanation of Table No. j. It should be understood 

 at the outset that it is not intended in this table to present 

 a scheme of classification that shall embrace all the differ- 

 ent kinds of igneous rocks that have been described and 

 narded. Many of these differ from common well-known 

 types in modifications of texture, or the proportions of 

 the minerals, or the presence of some other mineral in 

 relatively small quantity, and are to be regarded as 

 having the value of varieties. For simplicity's sake 

 only more common or important types are given for 

 illustration in each division, and varieties are treated 

 later in the descriptive part of the work. 



The rocks are divided into five large groups. A, B, C, 

 D and E, according to the nature of the feldspars that 

 they contain. In A these are dominantly alkalic, ortho- 

 clase, albite, etc. ; in B, alkalic and sodacalcic, that is to 

 say, mixtures of alkalic feldspars with plagioclase, as for 

 example orthoclase and andesine or oligoclase. Various 

 types of mixtures may occur, but the essential thing is 

 that lime is associated in the feldspars in notable quan- 

 tities with the alkalies. In C on the contrary the soda- 

 lime feldspars dominate over or replace the alkalic and 

 may run through oligoclase and andesine to labradorite. 

 In D the alkalic feldspars have practically disappeared 



^"Igneous Kocks; vol. II, p. 347, 1909. 



