622 



SOIENGE. 



[N. S. Vol. VIl. No. 175. 



able, however, that these processes will be 

 farther simplified before they will meet 

 with general introduction. I am informed 

 that several devices are being considered 

 which will enable the effect to be produced 

 by means of a single lantern. Various prin- 

 ciples involved in the forms of stereoscope 

 above discussed make it evident that such 

 a device is by no means impracticable. 



This eclectic summary of the progress of 

 invention in the field of stereoscopic vision 

 would seem to indicate that the interest in 

 this topic is undiminished and that the field 

 is still open for improvements and modifica- 

 tions which shall be useful in exhibiting the 

 principles which underlie the workings of 

 this truly psychological instrument. 



Joseph Jastrow. 



University of Wisconsin. 



CLASSIFICATION OF IGNEOUS SOCKS* 

 Peofessoe Mereill remarked at the last 

 meeting of the Geological Society that rock 

 species do not exist in the definite sense in 

 which this term is used in the organic 

 world. Probably no petrographer will deny 

 this conclusion. 



Admitting, then, that rocks are mineral 

 mixtures which may vary indefinitely, it is 

 clear that the naming of these mixtures 

 may be carried to excess. Let us, for ex- 

 ample, take the feldspathic lavas. These 

 may be divided into three great groups, the 

 alkali-feldspar lavas or trachytes, the oli- 

 goclase-andesine lavas or andesites, and 

 the labradorite-anorthite lavas or basalts. 

 These three groups may be written graph- 

 ically as follows : 



The demands of modern petrography, 

 perhaps, require the recognition of inter- 

 mediate groups which may be designated 

 by compound names. Thus a rock inter- 

 mediate between syenite and diorite may 

 be called a syenite-diorite. Such would be 



* Bead before the Geological Society of Washington 

 on Febrnary 9, 1898. 



the rocks called mozonite by Brogger. Be- 

 tween the three great groups of feldspathic 

 lavas above outlined there may be insti- 

 tuted three intermediate groups, as repre- 

 sented in the above diagram. One group 

 intermediate between trachytes and an- 

 desites may be called the trachyte-andesite 

 group ; that intermediate between the 

 andesites and basalts the andesite-basalt 

 group, and that intermediate between 

 trachytes and basalts the trachyte-basalt 

 group, or, as Washington has suggested, 

 trachydolerite, adopting a term in use in 

 Italy. The recognition of such intermediate 

 groups seems to me quite desirable, but if 

 the subdivisions are carried still further the 

 results are, perhaps, of questionable value. 

 To see how complicated the classification 

 may easily become, let us take the single 

 intermediate group of trachyte-basalts. 

 There are in Italy, in the Yellowstone 

 Park, near Buda-Pesth and in California 

 rocks which occupy this intermediate posi- 

 tion. These lavas have been studied by 

 skilled petrographers, and their excellent 

 descriptions, with the accompanying chem- 

 ical analyses, leave no doubt as to their ex- 

 act nature. 



The following names have been proposed 

 for different varieties : 



Trachyte-basalt group : 



Yellowstone Park 

 (Iddings) 



Italy 



(Washington) 



Buda-Pesth 



(Koch) 

 California 



Absarokite 



Shoshonite 



Banakite 



Trachydolerite 



Toscanite 



Vulsinite 



Ciminite 



-! Labradorite-trachyte 



Trachandesite 

 (Latite). 



Dr. "Washington, in one of his admirable 

 petrographic papers,* notes the close re- 

 semblance of his Italian lavas with those 

 of the Yellowstone Park, but nevertheless 



* Journal of Geology, Vol. V., page 363, 1897. 

 See also table on page 366. 



