MAGMATIC DIFFERENTIATION IN HAWAII 291 



a matter of pure differentiation. An inspection of the results so 

 far attained in Hawaiian petrography clearly suggests the nature 

 of the local primitive magma, namely, basalt, and shows the march 

 of magmatic differentiation in the main island, from that parent 

 species to the less voluminous rock-types which have so far been 

 discovered. 



The first part of this paper is devoted to a description of rock- 

 types forming part of a collection made by the writer in 1909. 

 From the facts won in that reconnaissance, and from those already 

 published in the writings of J. D. and E. S. Dana, of Lyons, Phillips, 

 Silvestri, Cohen, Mohle, Maxwell, C. H. Hitchcock, Brigham, 

 Button, Cross, and others, an induction has been made as to the 

 probable origin of the rock species in Hawaii. A brief statement 

 of the reasoning on which this tentative conclusion is based occupies 

 the second part of the paper. 



The four new rock analyses and the analysis of phenocrystic 

 olivine were made by Mr. G. Steiger, chemist of the United States 

 Geological Survey. For these excellent data the writer's sincere 

 thanks are due to him, and to Dr. G. O. Smith, the Director of the 

 Survey, who generously acceded to the request that this work 

 should be undertaken by the able experts of the government 

 laboratory. 



SPECIAL PETROGRAPHY 



Porphyritic gabbro of the Uwekahuna laccolith. — About two 

 hundred meters north of the Uwekahuna triangulation station, 

 the western wall of the Kilauean sink exhibits a patch lighter in 

 color than the average rock in the cliff. This patch is visible 

 from the Volcano House and the writer made an early visit to this 

 part of the sink. (See locality marked "1" in Fig. 1.) Nearing 

 the place, it was observed that the light-tinted rock was more 

 massive than the lavas above and below it. Large blocks of the 

 rock had fallen from the cliff and many were plainly seen to have 

 been derived from the lighter-colored mass, which was gabbroid 

 in habit. With a little trouble the writer was able to scale the cliff 

 for the vertical distance of about 20 meters, necessary to reach the 

 lower contact. There the gabbroid rock showed a distinctly 

 chilled phase in a contact shell several decimeters in thickness. 



