part 3] COMPOSITE SILL AT ]SrEWTO]S' ABBOT. 269 



determination ; but much of it is chloritized, with the development 

 of glassy-clear secondary albite. Quartz occurs in considerable 

 cpantity, both original and xenolithic ; ilmenite or pyrites re- 

 presents the iron-ores ; and secondary minerals are epidote and 

 calcite, the latter forming veins. The specific gravity is 2*59, this 

 low figure being due to the altered character of the rock. 



V. Summary and CoNCLrsiONS. 



The sill described in the foregoing pages is intrusive in the 

 Upper Devonian formation, the intrusive character being indicated 

 by the spotting of the slates. It has been shown that differenti- 

 ation has taken place, resulting in rocks which vary from picrites 

 through dolerites to bostonite. The picrite forms the base of the 

 intrusion, and has slight lateral extension : it has been partly re- 

 sorbed by the dolerite, Avith the formation of basic xenoliths in the 

 dolente and of xenolithic structure in the picrite ; the junction 

 is well defined. The dolerite forms the main mass, and the latest 

 products of the differentiation are the bostonite- veins. Mugearite 

 occurs as a modification of the dolerite. Quartz is an original 

 constituent, and has also been obtained from outside sources, 

 perhaps from Lower Devonian grits beneath the slates. 



Differentiation has not taken place in situ, as the junctions do 

 not shade one into the other, but are always sharp — except in the 

 case of the mugearite. 



The order of differentiation is of decreasing basicity, and hence 

 probably represents a plutonic phase, although the scale of the 

 intrusion is of the minor order. 



Stratification of the picrite and dolerite took place in the magma 

 reservoir, parts of the picrite were carried forward by the dolerite, 

 and the bostonite was of slightly later production, but earlier 

 than the final cooling of the sill, as it shows no selvage. 



The albitic final phase of differentiation suggests a Culm age 

 for the intrusion. 



EXPLANATION OF PLATE XL 



Eig. 1. Xenolith in dolerite (N 10 A). Summit Quarry, Knowles Hill 

 (Newton Abbot). From the central part of the xenolith. Glomero- 

 porphyritic aggregate of chlorite and serpentine after olivine ; augite- 

 plates at the lower edge. The texture of the olivine is subhedral, as 

 in the picrites. Ordinary light, X 30 diameters. (See p. 265.) 



2. Fine-grained dolerite, ' mugearite variation ' (N 14). Summit Quarry, 



Knowles Hill. Oligoclase-laths giving a trachytic aspect. On the 

 right edge is a small orthoclase, north-west of this two small 

 anhedral quartzes, while below is a small rhomb of soda-orthoclase. 

 Crossed nicols, X 35 diameters. (See p. 267.) 



3. ' Bostonite '-vein (N 9). Summit Quarry, Knowles Hill. Rhombic 



sections of chequer oligoclase-albite, with inclusions of chlorite. 

 Three crystals in the central upper part on the right show coarse 

 chequer- structure ; to the left of these are two rhombs of soda- 

 orthoclase. The central crystal shows albite-lamellae, nearly at 

 extinction. Crossed nicols, X 30 diameters. (See p. 266.) 



4. Soda-syenite segregation in albite- diabase, West Quarry, Trusham 



Station. This shows sections of albite and soda-orthoclase, which 



