THE BASIC MASSIVE ROCKS, ETC. 593 



entirely aphanitic, and all kinds tend to a porphyritic develop- 

 ment, carrying as phenocrysts oligoclase and more rarely labra- 

 dorite and auonte. Like the diabases mentioned above, the dia- 

 base-porphyrites are furnished with amygdaloidal upper portions. 

 In the few instances in which the olivine-bearing rocks have an 

 undifferentiated glassy base, they are called melaphyres, although 

 placed among the fine-grained diabases. 



The most of the basic rocks of the region are in the form of 

 interbedded sheets. Dykes are rare. When they occur, their 

 material appears to be diabase or diabase-porphyrite. It is rarely 

 coarse enough to be classed with the rocks called gabbro. 



In the Huronian areas on the other hand, large dykes of 

 coarse-grained gabbros 1 cut through the sedimentary beds, and 

 with these are intercalated thick beds of gabbro, and occasionally 

 a few thinner ones of diabase. 



Since Irving's general classification of the rocks in question a 

 few other publications have appeared in which the petrography 

 of small areas, and the descriptions of hand-specimens are treated. 



Messrs. Herrick, Tight and Jones 2 busied themselves during 

 one summer with a study of the rocks around Michipicoten Bay, an 

 arm of Lake Superior extending northeasterly into Canada. Their 

 paper contains but little with respect to the basic eruptives not 

 found in Irving's monograph. Dr. Wadsworth 3 has examined 

 some of the specimens gathered by the Minnesota Survey and 

 has divided the basic rocks into peridotites, basalts, including 

 gabbros, diabases, melaphyres, diorites and norites, peridotites, 

 and rocks regarded as altered andesites. All of Dr. Wadsworth's 

 descriptions are marked by exactness, but the conclusions based 

 upon them are rendered less valuable than they would have been 

 had Wadsworth himself not been compelled to depend upon others 



1 It will be shown later that most of the rocks called gabbro by Irving and others, 

 are not gabbros, but are coarse-grained diabases. 



2 C. L. Herrick, W. G. Tight and H. Jones : Geology and Lithology of Michi- 

 picoten Bay. Bull. Scient. Lab. of Denison Univ., Vol. II., Part 2, 1887, p. 120. 



3 Dr. M. E. Wadsworth : Preliminary Description of the Peridotytes, Gabbros, 

 Diabases and Andesytes of Minnesota. Bull. No. 2, Geol. and Nat. Hist. Survey of 

 Minn., 1887. 



