624 



SCIENCE. 



[N. S. Vol. VII. No. 175-.. 



some would be at variance with tlie set of 

 intermediate families proposed in the above 

 diagram. However, I understand Dr. Ran- 

 some to regard the feldspathic lavas as be- 

 ing more properly represented in a linear 

 series, that is to say, the andesites would 

 be regarded as a group intermediate be- 

 tween trachytes and basalts, and, following 

 this system, it would be impracticable to 

 institute any such group as the trachyte- 

 basalts. The only possible intermediate 

 group between the trachytes and plagio- 

 clase lavas would be trachyte-andesites. 

 Dr. Eansome, in a forthcoming bulletin of 

 the U. S. Geological Survey* on the trachan- 

 desites (latites), clearly states their relation 

 to the Italian and Yellowstone Park lavas, 

 and the position of these California rocks is 

 at once evident to the reader. However, 

 if we call all lavas composed of labradorite- 

 anorthite feldspars, basalts, the recognition 

 of a group intermediate between the 

 trachytes and basalts seems inevitable. 



While greater definiteness is unquestion- 

 ably desirable in the naming of rocks, it 

 seems but fair to the general geologist, and 

 even to many who have given some time to 

 the study of igneous rocks, that general 

 terms should be used in all such descrip- 

 tions, even if in subordinate paragraphs the 

 rocks are given more definite names. This 

 enables the reader at once to place approxi- 

 mately the rock. 



It is possible that many of the current 

 names for rocks could be improved by sub- 

 stituting a name derived from the most 

 prominent mineral which enters into their 

 composition. Tlius a syenite might be 

 called an orthosite, from the French term 

 orthose or orthoclase. An augite-syenite 

 would then be called an augite-orthosite. 

 Even a granite might be called a quartz- 

 orthosite, if in the use of compound terms 

 we recognize only those minerals which are 



* An abstract of tbis bulletin -will soon appear in 

 tbe Am. Jour. Sci. 



essential constituents and never use the 

 names of accessory minerals in this way. 

 There appears to be the greatest latitude in 

 this matter. Some rocks are called horn- 

 blende-andesites in which only occasional 

 hornblende needles are to be noted, and the 

 same statement may be made with varia- 

 tion as to the mineral with a great many 

 names. It appears to the writer that a- 

 hornblende-andesite should be a lava com- 

 posed, if sufficiently crystalline, chiefly of 

 oligoclase and andesine feldspars, with rela- 

 tively abundant hornblende. This matter 

 of keeping in mind the relative proportions 

 of minerals in naming rocks, if carried out 

 throughout the entire series of rocks, both 

 with granolites and effusive rocks, would 

 result in a name conveying at once a toler- 

 ably accurate idea of the composition of the 

 rock. 



The term basalt indicates nothing what- 

 soever to the reader as to what the rock is 

 made up of. If we use the French term 

 labradorite we have at once a definite idea 

 as to what the mineral composition of the 

 rock is. The term labradorite is used by 

 the French for basalts which contain no 

 olivine. The term olivine- labradorite could 

 be used for olivine-basalt. 



This method of deriving the name of the 

 rock from its mineral components is thus 

 not at all new. The andesites usually con- 

 tain the feldspar andesine. The group of 

 peridotites derives its name from the word 

 peridot or olivine. A mica-peridotite is 

 plainly a mica-olivine rock. An enstatite- 

 peridotite rock is plainly an enstatite-oli- 

 vine rock. There are now two terms used 

 for the latter. One is Harzburgite* and the 

 other Saxonite, and there has been much 

 discussion over which name should take 

 precedence. If we drop both terms we re- 

 lieve the memory and make it plain to every 

 one what the composition of the rock is. 



* Eosenbusch, ITikrosl-opische Physiographie dor 

 Massigen Gesieine, 1896, p. 355. 



