Fairbanks.] 



Geology of Point Sal. 



83 



themselves out from the magma." They state that no signs of 

 crushing are apparent in the rock. 



Bonney* noted a similar appearance in the lherzolite of the 

 Lac de Lherz, which he attributed to a " fluidal structure." 



McMahon| advanced the following opinion to account for the 

 foliation of the Lizard gabbros: "That the latter was produced by 

 'shearing motion' on the rock after its consolidation had advanced 

 considerably, but was still incomplete." He notes also a rapid 

 passage in some of the dikes from a foliated to granitic structure, 

 and attributes the latter to greater solidification. 



HarkerJ describes a mottled appearance in some hornblende 

 picrites due to the "mingling of dark and light patches or the sep- 

 aration of white spots on a black ground," and also a banding, as 

 follows: "This last-mentioned appearance seems to be caused by 

 the difference between bands alternately rich and poor in olivine, a 

 character noted by Reusch in the saussurite gabbros of the Bergen 

 district. There are even distinct bands less than an inch in width, 

 which under the microscope appear as veins entirely composed of 

 serpentinized olivine with much secondary magnetite." 



Teall§ in an article on the metamorphosis of the Lizard gabbros 

 considers the foliation the result of pressure or regional metamor- 

 phism, saying : "Is it an original or secondary structure? That it 

 is due to a differential movement in the mass after the separation of 

 the original constituents will I think be admitted on all sides. The 

 only question that can arise is whether the movement was in any 

 way connected with the intrusion of the rock." 



Geikie and Teall|| have given in detail the structure of some 

 Tertiary gabbros in the Isle of Sky. They recognize four distinct 

 varieties: "(1) dark, fine-grained granulitic gabbros, resembling 

 externally basalt rocks; (2) well-banded gabbros composed of irreg- 

 ularly alternating bands and laminae of the several constituent min- 

 erals; (3) coarse massive gabbros destitute of any banding; and (4) 



*Geological Magazine, Vol. XVI, p. 370. 

 ^ Ibid., Vol. XXIV, p. 76. 



JQuart. Jour, of the Geol. Soc, Vol. XLIV, p. 457. 

 ^Geological Magazine, 1886, p. 489. 

 HQuart. Jour, of the Geol. Soc, 1894. 



