Fairbanks.] 



Geology of Point Sal. 



77 



atively narrow mass of serpentine between two gabbro dikes, offer- 

 ing the least resistance to crushing, has been reduced to clay, and 

 contains nodules of a dike not seen in place in that vicinity. 



Relation betzveen the Magnesian and the Fcldspathic Rocks. — 

 The study of the rocks at Point Sal, taken together with what is 

 known of similar areas in other parts of the state, leads irresist- 

 ibly to the conclusion that there is a genetic relation between the 

 feldspathic and non-feldspathic types. It appears also that there is 

 a common sequence for at least some varieties of the rocks thus 

 associated together. The Point Sal occurrence differs chiefly from 

 others in the state in the great variety of rocks, as well as in the 

 presence of a comparatively large area of gabbro. 



The association of gabbro and peridotite is a very common one 

 in other parts of the world. G. H. Williams* says: "Olivine rocks 

 and serpentines are more or less extensively associated with most 

 of the large gabbro areas which have hitherto been carefully studied 

 and described (e. g., Hartz Mountains, Saxony, Lower Austria, 

 Scandinavia, etc.); and the Baltimore region proves to be no excep- 

 tion to this rule." In the first of his papers on the rocks of the 

 Cortland Series,f he says, after giving the classification of Professor 

 Dana, diorite, norite, diabase, gabbro, and peridotite: "This classi- 

 fication may perhaps be advantageously followed, provided it be 

 remembered that no sharp line can be drawn between the different 

 groups, but on the contrary every possible transition from each 

 group into every other occurs." Bailey^ has described basic peri- 

 dotitic separations from the gabbros of the Lake Superior region, 

 but they are local and belong to the same period of eruption. 

 Judd§ says of the gabbros and peridotites of Scotland: "Locally 

 both gabbros and dolerites are found passing into peridotites by the 

 gradual disappearance of the feldspar and the increase in the quan- 

 tity of olivine." He says also that segregation nodules of gabbro 

 are found in the peridotites, and vice versa, and also that "the 

 so-called 'segregation veins' of peridotite are seen traversing the 



*U. S. Geol. Survey Bull. 28. 



fAmerican Jour, of Science, Vol. 31, p. 27. 



{Journal of Geology, Vol. II, p. 817. 



iQuart. Jour, of the Geol. Soc, Vol. XLI, pp. 358, 359. 



