484 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 57. 



No. 1 and No. 2 tunnels, and on the level of the No. 2 tunnel it is 

 quite fresh, has a width of about 2 feet, and cuts cleanly across the 

 vein on this level and the level below. On the No. 1 level below the 

 tunnel one dike cuts cleanly across the vein, while the other is de- 

 flected along the vein for some feet and dies out. ( See fig. 2. ) Below 

 this level these two unite to form a single dike, which gradually 

 widens with depth. Two specimens studied came from the old 

 sheaveway above the inclined shaft, on the level of the No. 2 tunnel. 

 One specimen was taken from the center of the dike and the other 

 from the contact with the inclosing slaty rocks of the Burke forma- 

 tion. 



In the hand specimen the rock from the Marsh dikes is dark 

 gray in color and porphyritic from the development of numerous 

 bright brownish-black phenocr3^sts of biotite in an almost aphanitic 

 gray groundmass. The individual phenocrysts of biotite average 

 about 2 mm. in diameter and are not well crystallized. No other 

 minerals can be distinguished with the unaided eye. The rock 

 is quite fresh, but where altered it assumes a brownish color and 

 becomes friable, the groundmass being completely altered before 



Fig. 2. — Showing relation of dikes (A) to vein (B) on no. i level ob" Maksh Minb. 



the biotites are attacked. The dikes are cut by numerous joints 

 which appear to follow no regular system. The rock has a rather 

 marked tendency to cleave parallel to the walls of the dike, due to 

 a flow structural arrangement of the biotite phenocrysts. The ma- 

 terial from the border of the dike is finer grained; the biotites are 

 slightly smaller, averaging about 1 mm. in diameter, are somewhat 

 better crystallized, and are more markedly arranged parallel to the 

 walls, thus emphasizing the cleavage of the rock. 



Under the microscope the rock is seen to consist essentially ol 

 biotite and augite, with accessory apatite and iron ore in an un- 

 individualized glassy base. Among the secondary minerals may be 

 mentioned abundant sericite which has developed in the ground 

 mass and also probably opal, tridymite, and some zeolites which have 

 developed in the augite. The biotite is of the type characteristic of 

 lamprophyres, with intense pleochroism in tones of dark and light 

 brown, deep embayments, and dark colored resorption rims. It is 

 usually entirely fresh and the phenocrysts are remarkjvbly free from 

 inclusions. The augite, which equals or exceeds biotite in amount^ 

 occurs in large phenocrysts with rather poor crystal outlines which 



