LA N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 27 
- The actual minerals of this rock are, named in the order 
— of their abundance: Microcline, plagioclase, quartz, biotite, 
 hornblende, titanite, apatite, orthite with epidote, magnetite. 
— No calculation of the mode was carried out, because it is 
very uncertain as to the måfic constituents in such cases 
when both hornblende and biotite are present. The norma- 
tive plagioclase is Abgg. In a thin section the plagioclase 
showed, in symmetric sections, a maximum extinction angle 
I of — 7” which should give a value of about Abg. 
The occurrence of hornblende is most probably a conse- 
Hd quence of assimilation of the fragment rock which is a horn- 
blende-kersantite. That resorption has actually taken place, 
appears from the rounded forms of the fragments. 
Most of the other specimens from aplite dikes examined 
under the microscope were also found to contain microcline 
in excess over the plagioclase, like the analyzed example. 
I have not examined specimens from a sufficient number of 
—dikes to state how general is really this potassic character 
of the aplite, but certainly it is not invariable. Thus a speci- 
men from topographical point 18, collected by Mr. Viscont, 
showed quite other proportions: The rock consists chiefly 
of feldspars and quartz. Among the former, plagioclase is 
far more abundant than the potash feldspar and forms some- 
what larger grains which are idiomorphic. They contain 
— potash feldspar in antiperthitic intergrowth. The plagioclase 
shows only very small extinction angles, thus being probably 
near AÅbg, in composition. As a mafic mineral there are very 
E small quantities of pale green hornblende. 
Dikes of Melanocratic Rocks and 
Composite Dikes. 
2 At places dikes of dark hornblende-bearing rocks are 
fairly frequent on Sviatoy Noss. They accompany the aplite 
—— dikes, and sometimes have formed composite dikes with the 
latter. Most occurrences of the dark dikes were met with 
On the southeast shore of Sviatoy Noss. Another tract where 
