406 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



passes through a number of the dark hoodoos near the lower portion of 

 the dark lower zone. Looking down from the top, it is seen passing 

 through successive hoodoos from right to left on the same horizon, and 

 swinging, perhaps, one quarter of the way around the mountain. Its 

 dip or hade is down and out, similar to that of the platy parting, but at 

 a much more nearly vertical angle, so that the platy parting passes 

 through it (see figure 5). Its shape as a whole, then, is like a segment 

 of the surface of a truncated cone and it enwraps the mountain in a 

 partial way as a bulb is partly enfolded by one of its leaves. 



The thickness of the band varies from one to two feet, averaging about 

 18 inches. Cutting across the dark hoodoos, it forms a striking and con- 



Figure 5. — The white Band. 



spicuous feature, as may be seen in the sketch shown in figure 5. It was 

 at first supposed to be a dike, but a study of it showed that this is not 

 the case. It was found that there was no sign of contact between it and 

 the dark rock through which it passes. The grain continues all through 

 the same, but at a certain line the augite ceases, the feldspathic con- 

 stituents increase and make up almost the whole mass of the rock. 

 More convincing yet. as shown in the sketch in figure 5, the platy part- 

 ing, with the remarkable disk-like structure, passes through both rocks 

 alike, and hence it cannot be a dike, but must be an integral portion of 

 the original liquid mass before it crystallized and cooled. It thus repeats 

 on a smaller scale what has already been observed at the transition zone. 



