part 2] 



or SOUTHERN ETEE PENINSITLA. 



83 



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penetration in lit -par -lit fashion has resulted, the foliation - 

 jDlanes being planes of maximum, invasion. In man}^ cases, this 



intimate penetration parallel to 

 a pre-existing foliation has con- 

 verted the foliated amphibolite 

 into a hornblende-gneiss of 

 hybrid origin. 



In other cases, the 1 i t - p a r - 1 i t 

 intrusion is limited to the border 

 of the amphibolite, while the 

 central and often less markedl}^- 

 foliated area is free from rbj 

 injected material. By the flow- 

 ing movements of the acid 

 gneiss, many of the more rigid 

 amphibolite-inclusions have been 

 broken apart, the gneiss flowing 

 in to fill the intervening space. 

 This gneiss is usuall}^ of a coarse 

 and pegmatitic type, and is often 

 a true hornblende - pegmatite 

 which passes by an impercep- 

 tible gradation into the normal 

 granite-gneiss (see fig. 4, p. 84). 

 This structure is particularly 

 well seen in the gneisses of the 

 coast of Sleaford. The amphibo- 

 lites of this region show, perhaps 

 more characteristicalh^ than else- 

 where, the complete enclosure of 

 the amphibolites by an aureole 

 of pegmatitic coarse gneiss or 

 hornblende - pegmatite, which 

 has no definite junction with 

 the normal gneiss, but passes 

 imperceptibly into it. 



The amphibolite or horn- 

 blende-schist inclusions are often 

 so abundant, that a survey of a 

 clean platform of the £j:neiss 



shows bands of grey gneiss alter- 

 nating with dark amphibolite- 

 bands, and the appearance on 

 the wave-swept shore-line of the 

 southern coast of Sleaford is re- 

 markably conspicuous. These 

 bands of amphibolite are often 



of great length, measured in hundreds of feet, and their breadth 



is variable. 



