MICRO PEGMATITE IN PURCELL SILLS. 
31 
periphery and the centre of the dyke at the time of cooling and 
solidification. Harker 1 describes the basification of the Carrock 
Fell to the differences of temperature between the borders and 
the centre of the mass during solidification. Other interesting 
examples of this phenomenon are described by Pirsson . 2 
The absence of the basic upper contact zone in some of the 
Purcell sills is probably due to the high temperature of the 
intruding magma. In such cases, the initial temperature was 
so high that before it sank to a degree at which the basic con- 
stituents would tend to segregate towards the margin, an 
almost perfect separation of the granite (micropegmatite) took 
place. In this manner, no basic upper contact would be expected. 
The absence of this special layer also proves that differentiation 
according to gravity has played the leading role in the separation 
of the acid and basic material, and not differences in temperature 
between the contact and the interior. If such were the case, a 
symmetrical igneous body would result. 
SUMMARY. 
The Purcell sills represent intrusions from a single intercrustal 
reservoir of a series of magmas — acid magmas — which gave 
rise to composite sills whose rock types vary in the same sill, 
from a granite (micropegmatite) to a gabbro ; and basic magmas 
which gave rise to simple sills of gabbro. 
The reservoir may be assumed to have been stratified according 
to density, having a relatively acid portion collected in the 
irregularities and projections of the roof and grading downwards 
into more basic materials. 
Crustal movements would furnish fissures which would tap 
this reservoir at various levels. In this way a separation of the 
acid and basic materials of the reservoir would occur, so that the 
acid and basic materials would rise through separate fissures 
and spread out between the strata as sills. Some exotic material 
was gathered up from the walls of the fissures through which they 
> Harker, A., Q. J. G. S., vol. 50, 1894, p. 311; vol. 51, 1895, p. 125. 
2 Pirsson, L. V., 20th Ann. Rep., U. S. G. S., pt. 3, 1900, p. 563. 
Pirsson, L. V., U. S. G. S. Bull. 237. 
Weed and Pirsson, Amer. Jour. Sci., 4th ser., vol. 12, 1901, P. I. 
