W. S. Bayley — Soda-Granite, el Point, 277 



the red rock decreases until finally none is present. However, 

 when tin' red rook is wholly absent Contact action is yet often 



marked. One or even more 1 oi the contact /ones may he pres- 

 ent, hut that which indicates the most intense action lias disap- 

 peared. It is plain, then that the red rock as an independent 

 irruptive is inadequate to explain the whole of the contact belt. 

 On the other hand, the phenomena of distribution of the 

 various rocks are such as would he expected were the gabbro 

 the cause of the existence of the red rock and the contact 

 zone. If the red rock is a result of contact action we should 

 expect to find it between the gabbro and the outer members of 

 the contact belt — in greatest quantity where the other contact 

 effects are best exhibited, and in small quantity where the other 

 contact products are scant in amount. Where the effect of the 

 metamorphosing rock upon the quartzites is very slight, the red 

 rock should be absent 



The great differences in the intensity of the action of the 

 gabbro upon the surrounding bedded rocks may be readily 

 explained by supposing that the orifices through which the rock 

 was extruded were situated at those places where the con- 

 tact belt is widest, and the red rock is present in greatest 

 amount, In the neighborhood of these orifices, the intruded 

 rocks would have been subjected to the influences of the pres- 

 ence of a molten mass near them for a considerable length of 

 time. Where the contact products are not abundant, the mol- 

 ten gabbro may have escaped from its channel and thrust 

 itself between contiguous beds of the clastic rocks, as is known 

 to be the case in several of the localities where the contact effects 

 are slight. 



The presence of a contact belt around the gabbro on Pigeon 

 Point, while it is absent, in most cases at least, from around the 

 dikes of a similar rock abundant in other portions of the Lake 

 Superior region, needs explanation if the view that this rock 

 is the cause of the contact belt in the former locality is to be 

 accepted. It may readily be accounted for by the fact that, 

 whereas, the gabbro along the north shore of the lake is in the 

 form of simple dikes, the rock on Pigeon Point possesses many 

 of the characteristics of a boss.* 



That the gabbro upon the point is fully capable of produc- 

 ing contact effects is plainly shown by the existence of contact 

 phenomena at the falls of Pigeon River, and at other places 

 where there is no indication of the presence of the red rock 

 in the vicinity. 



The effect of the gabbro upon the quartzites and slates of 

 the point can be studied directly in the inclusions of the latter 



* Ot N. II. Winchell, Ninth Ann. Rep. Geol. and Xat. Hist. Survey of Minne- 

 sota, p. 63. 



