226 



SCIENCE. 



[N. S. Vol. V. No. 110. 



Dr. von Buchka has been appointed the 

 successor of Dr. Eugen Zell in the Imperial 

 Board of Health, Berlin, and has qualified him- 

 self as privat-docent in the University of Berlin. 

 The subject of his inaugural lecture being ' The 

 scientific basis of the newer development of 

 analytic chemistry.' 



DISCUSSION AND CORRESPONDENCE. 

 NOMENCLATURE OF METAMORPHIC LAVAS. 



In the gold belt of the Sierra Nevada there 

 are two very distinct sets of lavas. One of these 

 is Juratrias in age or older, and is separated by 

 a marked unconformity from a later set of lavas, 

 chiefly of Tertiary age. The older set of lavas 

 has been metamorphosed in varying degrees, so 

 that at some points their original nature is not 

 evident. Even where little altered they almost 

 universally contain secondary minerals, such as 

 epidote, zoisite, chlorite, uralite and calcite. 

 The general appearance of these rocks and their 

 mineral composition is, therefore, very different 

 from the correspondng set of Tertiary lavas. 



It is, therefore, very confusing to the general 

 public to use the same set of names for the two 

 sets of rocks. In the gold-belt region the older 

 series of lavas has been compressed and infolded 

 with the Juratrias and older sediments, which 

 are called the auriferous slate series, since they 

 contain by far the larger portion of the gold- 

 quartz veins of the Sierra Nevada. 



In many other parts of the world there are 

 similar broad distinctions to be made between 

 an older and younger set of lavas. It is very 

 important that some method should be devised 

 by which one may designate the fact that any 

 given lava originally corresponded to a given 

 type and at the same time express the fact that 

 it has undergone metamorphism. This has 

 been done in specific cases in various parts of 

 the world. Thus Dr. Bascom, with the rhyo- 

 lites expresses by the prefix apo- the fact that 

 the rock, originally a glassy rhyolite, has under- 

 gone devitrification. The prefix epi- has also 

 been used in some cases to express alteration. 

 Thus the rooks called the epidiorite have been 

 shown in many cases to have resulted from the 

 alteration of diabase. The term metadiorite 

 was used by Dana, and lately has been used by 

 Cross and the writer, for diorites formed by the 



alteration of other rocks, without reference to 

 their original character. None of these terms 

 has thus far been used in a general way to ex- 

 press alteration in all of the altered lavas. 



Dr. Bascom writes concerning the three pre- 

 fixes that have been mentioned, as follows:* 



"The prepositions meta, epi and apo, as pre- 

 fixes, all indicate some sort of an alteration. 

 Their exact force has been thus defined by 

 Professor Gildersleeve : ' Meta indicates change 

 of any sort, the nature of the change not speci- 

 fied.' This accords with the use of the prefix 

 by Dana in such terms as 'metadiorite' and 

 ' metadiabase.' These terms have been re- 

 cently revived to designate rocks ' now similar 

 in mineralogical composition and structure to 

 certain igneous rocks, but derived by meta- 

 morphism from something else.' Epi signifies 

 the production of one mineral out of and upon 

 another. This prefix has not been much used. 

 We find it in such terms as epidiorite, epigenetio 

 hornblende and epistilbite. Apo may prop- 

 erly be used to indicate the derivation of one 

 rock from another by some specific alteration." 

 It is evident that Professor Gildersleeve's 

 definition, that the prefix meta is the most log- 

 ical one to use to express, in a general way, the 

 metamorphic condition of altered rocks. Prob- 

 ably if the term had not already been used in 

 certain specific senses there would be no objec- 

 tion to its adoption for this purpose, and speak- 

 ing of meta-andesites instead of porphy rites ; of 

 meta-basalts instead of melaphyres, of meta- 

 rhyolites instead of quartz-porphyries, and of 

 meta-trachytes instead of orthophyres. This 

 would express not only at once the fact that the 

 rock was originally an andesite, basalt, rhyolite 

 or trachyte, but also of the fact that it had 

 undergone a metamorphism which would sug- 

 gest the presence of the various secondary min- 

 erals which are almost always found in such 

 altered lavas. 



The adoption of this system of nomenclature 

 is urged by the writer as a means of simplify- 

 ing the very burdensome and unphilosophical 

 series of names in use at the present time. 



H. W. Turner. 

 U. S. Geological Survey, 



January, 1897. 



* Journal of Geology, Vol. I., p. 827. 



