266 EDWARD STEIDTMANN 



alkalies and silica. In harmony with this, he finds very few cases of 

 porphyritic texture and very little difference in the time of crystal- 

 lization of the various constituents. In general, labradorite has 

 come out first and augite last in the dikes and flows, while in the 

 deep-seated intrusives, the order is reversed. He also regards the 

 development of olivine rather than enstatite or hypersthene as in 

 harmony with the position of the analyses with respect to his 

 hypothetical eutectic. 



The igneous rocks are all crystalline. If any glass has existed in 

 these rocks it has long since become devitrified. The following 

 varieties of texture are described by Lane: (i) ophitic, in which the 

 augite crystals occur as a cement, enclosing idiomorphic feldspars; 

 this texture characterizes the central portion of the basic effusives ; 

 (2) doleritic texture, in which the feldspar is much coarser than the 

 augite; (3) glomeroporphyritic or navitic, in which the feldspars 

 present a great range of size and tend to aggregate in bunches ; (4) 

 porphyritic hiatal, characterized by coarse feldspar phenocrysts 

 imbedded in a ground mass in which the feldspars are very small; 

 (5) amygdaloidal texture in which blow holes have resulted from the 

 escape of gases; (6) microlitic, a texture which develops around 

 amygdules; it is characterized by fine slender prisms of feldspars; 

 (7) vitrophyric, consisting of a glasslike ground mass in which a few 

 crystals are imbedded; it is always found within a few millimeters 

 from the margin; (8) the graphic texture, consisting of intergrowths 

 of quartz and feldspar; it is very common in the interstices of the 

 Bessemer gabbro and is also very abundant in felsite conglomerate 

 pebbles; (9) spheruhtic texture, found only in salic rocks; it is 

 characterized by small spherules of rock; (10) mozaic texture, con- 

 sisting of equigranular quartz and feldspar. 



The factors which determine the grain of igneous rocks are 

 stated by Lane to be the chemical composition, temperature, and 

 pressure of the magma. He presents a mathematical discussion of 

 the relation between grain and cooling. 



The mean annual temperature of the air on Keweenaw Point is 

 between 38° and 42° Fahrenheit. The temperature of the upper 

 mine levels at a depth of from 100 to 236 feet varied from 43° to 

 50°, and the average was nearly 43°. The rate of increase of 



