298 A. Wandhe — Study of Cape Neddich Gabbro. 



tially have satisfied the stresses involved and along this 

 break magma nnder a high hydrostatic head may have 

 risen. If the tensional stresses within the sediments 

 were not entirely satisfied by the ruptnre it seems 

 probable that a lateral creep of walls may have taken 

 place aided by the hydrostatic pressure of the ascending 

 magma. If the hydrostatic pressure of the magma and 

 the resistance to movement of the solid walls were still 

 unequally balanced the enclosing walls may have been 

 thrust apart until equilibrium ensued. The field rela- 

 tions indicate that such a thrusting apart of the walls 

 actually took place. After the intrusive had approxi- 

 mately attained its present size it may be that the contact 

 which in the first stage had been left jagged was now 

 smoothed off. Stoping and local assimilation, processes 

 indicated by included blocks and marginal hybrids, evi- 

 dently were responsible for this final smoothing of the 

 contact. 



Petrography. 



In speaking of this intrusive rock as a gabbro atten- 

 tion is called merely to the dominant marginal phase. 

 This stock in reality shows four distinct phases: (1) a 

 marginal or contact phase characterized by hypersthene 

 and approximating a quartz-norite in composition; (2) 

 gabbro; (3) and (4) two contrasting central phases, the 

 one characterized by ferromagnesian minerals and sug- 

 gesting cortlandite, the other characterized by plagioclase 

 feldspar and best described as an anorthosite. An opti- 

 cal analysis by the Rosiwal method gives the following 

 approximate mineral composition of each of these four 

 phases : 



Contact Gabbro Central 



Olivine 17.89 1.50 6.76 



Hornblende 10.01 21.42 .68 



Pyroxene 23.53 25.00 8.23 9.43 



Biotite 3.77 3.26 4.76 1.21 



Plagioclase 45.50 45.60 51.85 87.10 



Alkaline Feldspar . . . 1.17 .16 



Magnetite 3.62 13.82 6.79 1.55 



Apatite 2.97 



Hypersthene 2.33 .... .... .... 



100.78 99.19 99.81 100.13 



