408 PKOF. P. MARSHALL OX THE [Allg. I906, 



14. An de site. — There is a group of rocks, forming in places 

 large and important lava-flows, which in their mineralogical com- 

 position occupy an intermediate position between the basalts 

 and the trachytoid phonolites. The most typical of them 

 occurs in the lower portion of the Leith Yalley. It is a coarse- 

 grained rock, with conspicuous phenocrysts of felspar, and 

 occasionally of hornblende as well. In slices, the felspar is not 

 much twinned, but has the extinction-angle of andesine. The 

 hornblendes have a wide resorption-border ; there are smaller 

 crystals of brown augite, and some occasional pseudomorphs after 

 olivine. The groundmass consists of felspar-microliths, augite, and 

 magnetite. The rock is, therefore, considered to be a basic type of 

 augite-andesite. 



Inclusions of a plutonic nature are not uncommon in this rock. 

 They are always a coarse diorite, consisting of andesine and brown 

 hornblende in about equal quantity ; there is also some pale 

 brownish-green augite, large crystals of apatite, and a little 

 magnetite. The apatites are rather peculiar, for they are in the 

 form of short prisms, contain a great number of gaseous inclusions, 

 and have a faint violet tinge. Apatite with the same character is 

 found rather frequently in the andesite, and less frequently in other 

 rock of the district. 



Chemically, the andesite is rather poor in silica but relatively 

 high in alkalies, showing in these respects a relationship to the 

 trachytoid phonolites that are so abundant in this district. 



SiO- 



A. 



Per cent. 

 51-36 



B. 



Per cent. 

 51-46 



Ti0 2 





0-32 



A1 2 3 



Fe"0, 



15-98 



7-64 



20-26 

 4-64 



EeO 



5-10 



3-56 



M11O 





0-32 



CaO 



6-50 



9-55 



MgO 



Na 9 



1-84 



4-26 



3-16 

 4-29 



K 



PlO 



3-97 



0-42 



2-47 

 0-57 



H o 



3-60 



034 









Totals 



100-67 



100-94 



A=Andesite, Paper-Mills, Leith Valley, Dunedin (N.Z.). Anal. P. Marshall. 



B = Hornblende-anc!esite with augite, Bogosloff I. (Alaska). Eosenbusch, 



'Elemente der Gesteinslehre ' 2nd ed. (1901) p. 307, No. 166. 



The amount of iron is high and alumina correspondingly low, as 

 in most of these rocks. The substantial agreement in the analyses 

 justifies the classification of* the rock with the augite-andesites. 



An amphibole from the andesite was also analysed, with the 

 following result (A) : — 



