78 



University of California. 



LVOL. 2. 



gabbro, and similar veins of gabbro are found intersecting the peri- 

 dotite masses. The relations of the gabbros and peridotites in the 

 Western Isles of Scotland seem to indicate that, in the heart of these 

 old volcanoes, the feldspar, olivine, and augite tended to segregate 

 in certain cases into masses of various dimensions, and that these 

 masses were, after consolidation, fissured again and again, the fis- 

 sures being injected by different portions of the magma, which were 

 still in a more or less plastic condition." Bonney,* in describing an 

 area of troctolite and serpentine in Aberdeenshire, says that the 

 feldspathic rock is the later. He quotes Judd as observing a 

 sequence of serpentine, troctolite, and gabbro. In notes on some 

 Ligurian and Tuscan serpentines hef describes the occurrence of 

 serpentine and gabbro together in a number of places and expresses 

 the opinion that it can hardly be a coincidence. BonneyJ also 

 notes, in describing the serpentines of Ayrshire, dikes of diallage 

 rock cutting serpentine, and gabbro cutting both in turn. 



From these references it will be seen that the association of 

 feldspathic and magnesian rocks with intermediate types is a com- 

 mon one. There must certainly be a genetic relation, as suggested 

 by several writers. There can be no question that at Point Sal all 

 the extreme variations and successive intrusions had one common 

 parent magma. 



The Banded Structure. — The remarkable banded and gneissic 

 structures shown in many portions of these rocks are among their 

 most interesting and peculiar features. Although the banding is 

 of course absent from the large area of altered dunite and from a 

 portion of the gabbro at the western end, yet it is such a common 

 phenomenon in almost all facies of the mass, that it deserves a care- 

 ful description. Two fairly distinct divisions may be made, (i) 

 that in which the banding or gneissic structure is due to the more 

 or less perfect segregation of the different components in bands in 

 a fairly homogeneous rock, and (2) that which is due to the string- 

 ing out in long dike-like strips of one or more minerals foreign to 

 the body of the rock, as that of the feldspar and pyroxene bands 



*Geological Magazine, Vol. XXII, p. 441. 



\Ibid., Vol. XVI, p. 370. 



tQuart. Jour, of the Geol. Soc, 1878, p. 778. 



