820 DE. F. DIXEY OX THE [vol. Ixxviil, 



Assimilation. — An interesting feature of the Sierra-Leone 

 complex is the great amount of absorption suffered by earlier rocks 

 when invaded by later intrusions ; the absorption of older norite 

 by younger norite and of both older and younger norites by beer- 

 bachite are examples of these reactions. AVith regard to th& 

 norites themselves, this absorption is the more remarkable, in that 

 the composition of the later rocks differed but slightly from that 

 of the earlier rocks. Cases have been described in -which a basic- 

 magma has been modified by the absorption of an acid rock,i or,. 

 more frequently, in Avhich an acid magma has been modified by the 

 absorption of a basic rock : as, for example, the Lizard Kennack 

 gneiss ~ and the ' marscoite ' of Skye ^ ; but a basic magma has 

 rarely been known to absorb basic rocks. Even where a basic- 

 magma has mvaded an ulti-abasic rock, as where the basic rocks- 

 of -Rum^ and Skye -^ break through and invade the ulti-a basic- 

 rocks, the older rock has simply been broken up and finally involved 

 with the newer rock to form an intrusion -breccia, while little or no 

 corrosion has taken place. As may be expected, however, a magma 

 has sometimes been known to absorb a rock of similar composition 

 locallv, as where the Skve s^abbro has ' to a limited extent actuallv 

 fused and incorporated' basaltic lavas. As a general rule, how- 

 ever, it is not to be expected that a magma should absorb readily 

 a rock of similar composition, since there is little scope for mutual 

 chemical reaction, and the invading ma^ma would carry little, if" 

 any, excess of heat.^ 



The unusual extent to which absorption has proceeded in the- 

 case of the Sierra-Leone norites is ascribed chiefly to the gi-eat 

 depth at which the action took place ; this would result in a high 

 initial temperature for the invaded rocks, and would also prevent 

 the heat of the intrusions from escaping, therehy allowing a great 

 length of time during which the reactions could proceed. More- 

 over, there is considerable evidence to show that many .of the 

 numerous later intrusions of norite are all parts of one or more 

 large intrusions, of which only the irregular upper portions are at 

 present visible. 



(7) Weathering and Liireritization of the Xorite. 



T\^iere the products of weatherino* are unable to accumulate, 

 the highly-resistant magnetite stands out in sharp relief on the 

 surface of the norite, which in the majority of cases consists largeh^ 

 of felspar, since the pyroxenes disintegrate most easily. Along the 

 foreshore the norites rich in iron weather to a smooth surface 



^ As, for example, Purcell Sills ; see E. A. Dalv. Mem. 38. Geol. Surv. 

 Canada, 1912. pt. i, p. 248. 



- ' The Geology of the Lizard A: Meneage ' Mem. Geol. Surv. 1912. p. 132. 



•^ ' The Tertiary Igneous Rocks of Skye ' Ibid. 1904. p. 183. 



-• Ihid. p. 69. 



5 Ihid. p. 64. 



•^ A. Harker, • The Xatui-al History of Igneous Eocks ' 1909, p. 3.57. 



