34 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 66, 



mineral, but it is fibrous and mixed with secondary minerals as though itself 

 secondary. Biotite is rare and in most cases secondary. * * * 



Possibly (the name) granophyre is appropriate for most of the rock. 



In the sill in the eastern part of the city there is a remarkable example of 

 perfect gradation from diabase to red rock. The diabase is of ordinary type, 

 with a finer contact phase at the base. It is exposed almost continuously for 

 a width of a mile, equivalent to a thickness of several hundred feet. The dia- 

 base grades up into a red-rock zone of smaller thickness and less regularity, 

 though a belt may be followed for several blocks. It is noteworthy that v/hile 

 the sill must be nearly 1,500 feet thick, the conspicuous gradation zone is less 

 than 50 feet thick, from black diabase to intensely red granophyre. 



A somewhat different gradation is observed in Lincoln Park and near the top 

 of the inclined railroad to Duluth Heights. In these places it is possible to select 

 samples showing all stages between gabbro and red rock, but the relations are 

 not those of a regular zone. The upper part of the banded gabbro shows many 

 local patches with interstitial red granophyre, grading into dike like stringers 

 and patches of red rock of complex form and relations. Many of these stringers 

 with sharply defined walls can be traced along their length into less sharply 

 defined markings and finally grade imperceptibly into the black gabbro which 

 formed the walls a few feet away. Both the gabbro and the red rock intrude 

 the roof, sometimes in the same crack, sometimes more distinctly. Although a 

 considerable part of the red rock is so much later in time of solidification that 

 it could intrude the gabbro, the texture of the red rock is coarse up to its con- 

 tacts and grades into that of the gabbro without a break, indicating that they 

 were about equally hot. The irregularity in form of the stringers may also be 

 a sign that the gabbro was not wholly solid. Such a relation may be described 

 as that of an aplite. 



Similar relations of gabbro to red rock, both gradational and aplitic, are easily 

 traced for many miles along the belt at the northeast end of the gabbro in Cook 

 County, w here the combined thickness is so reduced as to make the mass more 

 like a sill, and the red rock constitutes a larger portion of the intrusion than at 

 Duluth. The same relation may be expected in the central, thicker part of the 

 gabbro mass, but this has not been mapped in detail as yet. 



A third gradation from red rock to gabbro is that in the j^egmatites near the 

 base. 



All three of these occurrences of red rock and gradations would seem from 

 field studies to be clearly attributable to a differentiation * * *. The sev- 

 eral occurrences may all be explained by supposing that the original magma 

 contained some vapors under pressure and that these tended to separate and 

 escape from the main magma bearing with them those acid and alkaline con- 

 stituents for wluch they seem to have a special affinity. The accumulation of 

 a definite upper zone of red rock would then be the result of a quiet rise of the 

 lighter vaporous separate under an impervious roof. The aplitic areas near the 

 top would be similar gravitative separates, disturbed by some movements at 

 about the time of solidification. The pegmatites and aplites below would be 

 located not so much by gravity as by simple vaporous tension; the lighter sep- 

 arate, being more fluid, might penetrate cracks on any side of the magma cham- 

 ber in advance of the main magma. 



Bowen^^ has discussed the crystaUization of basaltic magmas with 

 especial reference to the frequent association of diabase and grano- 



>' Norman L. Bowen, The later stages of the evolution of the igneous rocks. Journ. Geol., Supple- 

 ment to vol.23, 1915. 



I 



