Pirsson and Washington — Geology of New Hampshire. 447 



the section exactly explains it, and the analyst is confident that 

 it is not to be attributed to admixture of magnesia. 



In comparing the composition of this rock with similar ones, 

 as seen in the table, the predominance of soda over potash, 

 producing as it does an amount of plagio-clase equal to the 

 alkalic feldspar, causes a certain difficulty, if one pays atten- 

 tion to the older systems of classification. This may be seen 

 by referring to the analyses included under the subrang of 

 lassenose in the tables of analyses recently published by one of 

 us.* One sees under this subrang, which includes rocks of a 

 similar chemical composition, types which have been called 

 granites, syenites and quartz diorites, as well as rhyolites, 

 dacites, andesites and trachytes, indicating the middle position 

 which such a magma holds. 



Classification. — In the quantitative system the norm of the 

 rock calculated from the molecular ratios is as follows : 



Qz... 30-72 30-72 >| 



Or... 12-23 \ Sal _ 95-15 _ ^^ Persal- 



Ab .- 34-58 I 60-15 } 95-15 Fem " 4"20 " ~ ane > 1 



An .. 13-34 ) Q_ __ 30-72 _ Britan- 



Co._. 4-28 4 .08 J F"~ 60-15 ~ ' nare, 4 



Hv._ 2-9^ 



K 2 0' + Na 2 _ 88 _ Toscan- 



""CaO"" "48 -1 * ' are, 2 



ko' 22 



H 2 0_. -65 NaO' = 66 = °' SS > Lassenose , 4 



Mt __ '46 \ 4.20 4-20 



II ... -7( 



Total, 100-00 pi ace formula, I, 4, 2, 4. 



The extra alumina above that needed to produce feldspars 

 is shown above by the production of 4'28 per cent of corundum 

 among the normative minerals. Tiie texture is granular, the 

 micas are not present in notable amount and the mode is there- 

 fore normative. If it is desired to note the small amount of 

 biotite present the rock is therefore a biotitic-grano-lassenose. 



In the prevailing qualitative systems it is somewhat difficult 

 to place this rock. It is a marginal facies of a syenite mass 

 with aplitic habit showing some fluidal structure, and it 

 stands mineralogically exactly on the line between the granite 

 and diorite families. If we determined it as an aplite from 

 its minerals and texture and used Erogger's definition of 

 the monzonite family, it would be a quartz monzonite aplite or 

 adamellite aplite. + ' Or it might be called a grano-diorite 

 aplite, using grano-diorite, as many have done, to signify a 

 transition rock between the granite and diorite families. 



*Chem. Anal. Ign. Eocks. TJ. S. Geol. Surv.. Prof. Paper 14, 1903, p. 173. 



fBrogger, Predazzo. Vid. Selsk. Skrift. M-N. Kl. 1895, No. 7, p. 60. 



