A TYPE OF IGNEOUS DIFFERENTIATION 635 
which are characteristic of the contacts’ of diabase with acid rocks. 
The association is too striking to be thus ignored. At Duluth the 
case is illustrated by a very narrow granophyr zone at the base of 
an extrusive diabase north of Short Line Park, where it overflowed 
a quartzose sand. Where such flows rest on the other flows no 
Fic. 7.—Aplitic stringers of red rock in the Duluth gabbro, near the top 
such granophyr has been detected. Again, at Pigeon Point a 
gabbro seemingly related to the Duluth gabbro has inclusions of 
quartzite which are characterized by a complete inclosing shell of 
red rock.t The composition of the red rock in these cases is not 
such as can be derived from the sediment, nor from the average 
t'W.S. Bayley, ‘“‘The Rocks of Pigeon Point,” U.S. Geol. Surv. Bull. r09, p. 101; 
R. A. Daly, “‘The Geology of Pigeon Point,” Am. Jour. Sci., XLII, 423. 
