82 



University of California. 



[Vol. 2. 



ties of the peridotite are probably due to the mixing of two partially- 

 differentiated magmas. They have not been subsequently intruded, 

 for no sharp line of contact is visible, and the darker portions shade 

 off into the serpentine. This is illustrated along the base of the cliffs 

 at Point Morrito. Below the olivine-pyroxene rocks there are 

 parallel banded layers, which, although differing from each other 

 considerably and appearing on first examination to be dikes, yet 

 show more or less blending across the strike. 



Banding of the kind observed at Point Sal is a common charac- 

 teristic of the deep-seated basic rocks in many parts of the world. 

 Prof. A. C. Lawson* says of the anorthosites of Beaver Bay, north 

 shore of Lake Superior: "The original allotriomorphic granular 

 structure has not been disturbed, and it is highly improbable that 

 the banding is in any way associated with shearing action after the 

 final consolidation of the rock. It seems to the writer to be essen- 

 tially due to some local chemical differentiation associated with 

 movement in the thickly viscous magma prior to crystallization." 



Baileyf mentions a banding in the gabbro of the Lake Superior 

 region. He describes it as being confined to the peripheries of the 

 great gabbro area. " The thickness of the bands ranges from 20 or 

 more feet down to the fraction of an inch only." 



Banding of the type under discussion is very prominent in many 

 portions of the British Islands, and the English geologists have 

 given a great deal of study to the question. Bonney and McMahonJ 

 describing the crystalline rocks of the Lizard District note a band- 

 ing or foliation in the serpentine. They say: " In the opinion of the 

 authors the structure can only be explained as a fluxion-structure, 

 that is to say, as being the result of traction acting on either an 

 imperfectly blended mixture of two magmas, differing slightly from 

 each other in composition, specific gravity, or fluidity, as in the 

 case of the banded felsite or rhyolite, or on a mass in which com- 

 plete crystallization has been arrested by subsequent motion, at a 

 time when only a portion of the constituent minerals had separated 



*Bull. No. 8, Minn. Geol. Survey, p. 15. 



tjournal of Geology, Vol. II, p. 817. 



iQuart. Jour, of the Geol. Soc, Vol. XLVII, p. 474. 



