REPORT OF TEE CHIEF ASTRONOMER 245 



SESSIONAL PAPER No. 25a 



Analysis of hybrid rock in gabbro sill. 



SiO, 54-02 



Ti0 2 1.95 



ALA 1208 



Fe,0 3 6-85 



FeO , 5-61 



MnO -09 



MgO 282 



CaO 14-63 



SrO tr. 



Na 2 .60 



K.,0 -14 



H„0 at 110°C -06 



H 2 above 110°C -62 



P 5 3 " .21 



C0 2 -19 



99-87 



Sp.gr 3-141 



The alkalies appear to belong, wholly or in largest part, to the hornblende r 

 the alumina, ferric iron, and lime to the epidote and hornblende. The epidote 

 has all the appearance of a primary mineral. In any case it has not been 

 derived through ordinary weathering, for the rock is strikingly fresh. 



The composition of the shell is evidently anomalous and represents a 

 double effect. On the one side, the abundant quartz and probably part of the 

 alkaline constituent in the hornblende represent material dissolved from the 

 block; on the other side, the special abundance of the amphibole, to the appar- 

 ently entire exclusion of soda-lime feldspar, shows that the block formed a 

 centre around which the amphibole, as one of the earliest minerals in the magma 

 to crystallize, segregated. As the amphibole substance was osmotically trans- 

 ferred into the quartzite block, so the quartz substance was diffused outward 

 into the magma. The shell has clearly not the composition expected through 

 the mere solution of the quartzite; the actual composition has also been con- 

 trolled by the concentration of the basic hornblendic material around a foreign 

 body. The latter may have acted after the manner of the crystal introduced 

 by the chemist into a saturated solution so as to produce crystallization through 

 ' inoculation.' 



In other words, magmatic assimilation and differentiation are both illus- 

 trated in the history of this shell of mixed material about the quartzite block. 

 It is, nevertheless, certain that the sill magma as a whole was acidified by the 

 solution of this block and still more by the solution of others now invisible 

 because completely dissolved. 



The phenomenon of the partial digestion of xenoliths is quite familiar at 

 intrusive contacts; its significance is only properly appreciated if one remem- 

 bers that the visible effects of digestion have but a small ratio to the total 

 solvent effects wrought by the magma in its earlier, more energetic, because 

 hotter, condition. It is not a violent assumption to consider that many 

 quartzite blocks have thus been completely digested in the original gabbro- 



