216 PROCEEDINGS OF SECTION OC. 
phenomenon, I may quote the mixed dykes of the Christi- 
ania district, Sweden, which have been fully described by 
W. C. Brogger (') and J. H. L. Vogt. (*) The central por- 
tion of these dykes is composed of a normal mica-syenite 
porphyry, but towards the margin there is a gradual passing 
over into a rock (kersanite) of more basic character. Vogt 
shows that the percentages of the component minerals in 
the dykes in the centre and at the margin vary in the fol- 
lowing manner : — 
Per cent. Per cent. 
Pyrite increases from 0°5 in centre to 2-3 at margin 
Apatite 4: 0°51 5 1°44 a 
Magnetite es 1-2 fe 10-12 i 
Mica i 5°10 “e 20-25 
Plagiuclase xe 40) (oligoclase) ,, 60-70 (labrador and by- 
townite) 
Orthoclase decreases from 30 53 Nil. 
Quartz an 10 x Nil. 
This proves that the rock at the margin of the dyke is 
much richer in the minerals belonging to the first period of 
crystallisation (pyrite, apatite, and magnetite), and, to a 
lesser degree also, the ferro-magnesian silicates belonging to 
the second period of crystallisation (in this case mica), than 
the rock at the centre. Moreover, the acid plagioclase in 
the centre of the dyke is replaced hy basic plagioclase at 
the margin, and orthoclase and quartz which are abundant 
in the centre of the dyke are absent at the margin. Series 
of analyses have been made right across these dykes, and 
have proved that the alteration in composition is quite 
gradual. 
Similar occurrences have been recorded by a _ great 
number of investigators in various parts of the world, so 
that the example cannot be regarded as an isolated case, but 
as an instance of a universal phenomenon. 
Although the concentration of the basic material at the 
margins of the eruptive mass appears to be the general rule, 
this is not always the case. Sometimes the basic material 
is concentrated in the centre, and sometimes the various 
rock types are irregularly distributed. Whatever the cause 
of magmatic differentiation may be, it is certain that the 
migration of the material has taken place before any of it 
has solidified: ths has oe proved in part Ey, microscopic 
(') Die Mineralien der Syenitpegmatitginge, &c. Z. f. Kryst, 
&c., 1890. 
(*) Zeitschrift fur Practische Geologie, Jan., 1893. 
