SKETCH OF 44GINA AND METHANA. 149 
greater or less fusibility of the rocks in question, their fusibility 
being in turn directly dependent on their chemical composition. 
Lack of space, however, forbids further remarks on this subject, 
and the examination and elaboration of the suggestion made 
above must be left for another place. 
Mineralogically the rocks of this region do not show, on the 
whole, very gradual transitions from one species to another, 
many of the groups being sharply separated from all the others, 
as in the cases of the hornblende-andesites and hornblende and 
biotite-dacites. On the other hand there is among the members 
of the pyroxene-andesite group a gradual transition from the 
augitic to the hypersthene end, and some other cases occur, as 
will have been noticed. 
The basic rocks are decidedly more pyroxenic than the acid 
ones, and of the two members of this family present augite shows 
a decidedly greater fondness for the most basic rocks than the 
hypersthene, which is one of the main components of a large 
group of dacites; it will also have been noticed that the hyper- 
sthenic andesites are more acid than the augitic. Hornblende is 
the most prominent ferro-magnesian silicate of the acid rocks, 
the green being especially characteristic of these, while the brown 
variety is usually associated with augite. Biotite is not very 
abundant, but occurs in the most acid rocks, either alone or with 
hornblende. Magnetite does not show any well defined prefer- 
ences, but in accordance with the law just enunciated it usually 
occurs in larger crystals in the more acid rocks. 
CHEMICAL COMPOSITION. 
The analyses* presented in Table I. were made for me by 
Dr. A. Rohrig of Leipzig, while Dr. H. Lenk of the University 
of Leipzig had the kindness to make the specific gravity 
determinations. Care was taken to select the most representa- 
tive and least altered pieces, though in some cases it was 
impossible to find quite fresh specimens. The figures in small 
«TiO. was determined as the residue left on volatilization of the silica with hydro- 
fluoric acid. H-.0 is the loss on ignition. 
