30 
MUSEUM BULLETIN NO. 2 
in the irregularities in the sill roof. Where the quartzites are 
in contact with the gabbro, very little metamorphism has taken 
place. 1 This explanation also accounts for the absorption 
of the quartzites by the red rock. The red rims around the 
quartzites in the gabbro may be red rock which has frozen to 
the quartzite. The blocks with the red rim then sank into the 
gabbro zone and were caught when the sill solidified. 
The composite sills, as described above, sometimes have a 
basic upper contact. This basic layer has a specific gravity 
of 2-966 and hence cannot represent the quickly chilled original 
magma, for in a sill which is half granite (micropegmatite), 
the specific gravity of the original magma after solidification 
and cooling would be 2-80. Hence there has been some differ- 
entiation of the basic elements to the cooler part of the magmatic 
chamber followed by differentiation under the action of gravity. 
A similar phenomenon of differentiation towards the cooler 
parts of a magmatic chamber has been described by Lawson. 2 
In this case, the intrusive mass is a dyke 150 feet wide with 
the following variations in chemical composition. 
I 
II 
S 1 O 2 
47-83 
4-57 
30-28 
6-72 
4-32 
trace. 
1- 30 
2- 19 
2-05 
57-50 
5-07 
23-44 
5-62 
2-76 
0-45 
2-01 
2-02 
2-25 
Fe 2 Os FeO 
CaO 
MgO 
KtO 
Na 2 0 
p 2 o 5 
Loss on ignition 
S.G 
3-028 
2-856 
I. At the contact of the dyke wall. 
II. At 75 feet from the contact, the middle of the dyke. 
These two extreme types grade into each other and are con- 
sidered as differentiates from an original homogeneous magma, 
differentiation being due to the difference in temperature at the 
1 Bailey, W. S., U. S. G. S., Bull., 109, p. 105. 
2 Lawson, A. C., Am. Geol., vol. 7, 1891, p. 153. 
