THE KEWEENAW AN. 



405 



were made for Dr. Bayley iu the Survey laboratory, and were reported by 

 him in tlie papers above referred to. 



No. 8589 contains a large proportion of diallage and olivine, while 

 No. 8786 is more nearly of the average composition of the entire mass. 



Analyses of Duluth gahhro. 



I 

 Constituent. 



SiOj 



TiOj.., 



PA 



AlA 



CrA 



FeO 



FeA 



NiO 



MnO 



CaO 



MgO 



K^O 



Na,0 



HjOatlOS" .... 

 H2O above 105° 



Total 



PETROGRAPHIC CHARACTERS OF THE LOGAN SILLS. 



Macroscopic characters. — The rocks forming the Logan sills are nor- 

 mally black, medium- to coarse-grained rocks, although varying to fine- 

 grained aphauitic facies upon the edges of the sills. Occasionally the rock is 

 a porphyry, with the feldspars as the phenocrysts. Some of the pheno- 

 crysts reach 4 inches in length, but they are normally smaller, ranging 

 from 1 inch to 2 inches. Very frequently we find these feldspars collected 

 into large masses which are made up almost entirely of these )ninerals. 

 Such areas usually possess as the result of weathering a light gray or 

 almost white color. The relatively slightly altered masses resemble very 

 closely in appearance the auorthosite of the Duluth gabbro mass. As will 

 be seen from the description given below, the sills are formed of rocks which 

 have the composition and characters possessed by the modern dolerites, and 

 they are here called dolerites. 



