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THE BASIC MASSIVE ROCKS, ETC.
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entirely aphanitic, and all kinds tend to a porphyritic development, carrying as phenocrysts oligoclase and more rarely labradorite and augite. Like the diabases mentioned above, the diabase-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 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 Michipicoten 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.
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