ADDRESS

BY

PROF. N. H. WINCHELL.

VICE PRESIDENT, SECTION E.

THE CRYSTALLINE ROCKS OF THE NORTHWEST.

I DESIRE to call the attention of Section E to some of the interesting problems that beset the geologist who undertakes to study the crystalline rocks of the Northwest, and especially that part of the Northwest which is included in the state of Minnesota. Until very recently it has been the practice of geologists, almost without exception, to refer every crystalline rock in the Northwest either to the Huronian or to the Laurentian. Thus, when the survey of the state of Michigan was reinaugurated in 1869, the geologists of the upper peninsula were compelled to choose between a confession of their inability to establish the age of the rocks they were studying and the adoption of some of the recognized designations. In Wisconsin the case was similar, with the additional fact that the Michigan geologists were collaborators. The same was true again in Minnesota. What more natural than that the Michigan and Wisconsin rocks should be found to extend, with nearly the same features, into the State of Minnesota, and that their familiar names should at once be applied to them?

But when on more careful examination, both in the field and in the literature of the crystalline rocks, and over a wider extent of territory, and especially in the light of more recent researches in New England, New York, Pennsylvania and Canada, it is found that the nomenclature is imperfect, and furnishes but a tottering scaffold to support the workmen of a great and ever-spreading structure, we are thrown into such difficulty and doubt that we are prone either to reject the old scaffold and build anew, or to clear away the accumulated rubbish about the foundation and examine on what basis the old one stands. To-day, however, we intend to do neither of these, but rather set forth a few of the incongruities and difficulties of the actual situation.

We are indebted, unquestionably, to the geologists of Michigan and Wisconsin for the most exhaustive and satisfactory description of the crystalline rocks of the Archæan age that has yet been published in America. In order that some of the difficulties of the situation may be made clear, I desire to review concisely the broad stratigraphic distinctions of the crystalline rocks that have lately been studied in Michigan, Wisconsin and Minnesota. By the aid of the published results of the surveys of Brooks, Wright, Irving, Rominger, Pumpelly and others, a generalized statement can be formulated. To these I shall add such published results and unpublished field observations from Minnesota as may be furnished by the survey of that state, in order that the scheme may cover correctly the crystalline rocks of the entire Northwest.

Omitting the igneous rocks, which in the form of dikes cut through the shales and sandstones of the Cupriferous formation and are interbedded with them in the form of overflows, we may concisely arrange the crystalline rocks, disregarding minor differences and collating only the broad stratigraphic distinctions, in the following manner in descending order:

There are six groups:

FIRST GROUP.

Granite and gneiss with gabbro.-This group is represented in Minnesota by the gabbro and red syenite at Duluth, and by the extension of this range of hills northeastwardly nearly to the international boundary. Its thickness is unknown, but certainly reaches several hundred feet. The outcrop of red granite near New Ulm, lying under the conglomerate and red quartzite, is probably in the southwestward line of extension of this group. This group is represented by No. xx southwest of lake Michigamme, by No. xx at Menominee and by No. 1 and 1a at Black river.

SECOND GRoup.

Mica schist. This group consists of schists that are micaceous and often staurolitic as well as garnetiferous. It can be seen in Minnesota on the Mississippi river at Little Falls, and at Pike rapids. The schists are variously associated with beds and veins of granite and gneiss. This is No. XIX at Marquetté, XVII to XIX

at Menominee, xx to xxII at Penokee, and has a maximum thickness of 5000 feet.

THIRD GROUP.

Carbonaceous and arenaceous black slates, and black mica schists. -These sometimes pass into roofing slates, with beds of iron ore, quartzite and diorite. This group includes the black slates of the Animikie group in northern Minnesota, of Knife lake, and Knife portage on the St. Louis river, and carbonaceous slates lately reported near Aitkin on the Mississippi river. It includes Nos. XIV to XVII at Marquette, Nos. VI to XVII at Penokee, and Nos. xv and xvi at Menominee. Thickness 2600 feet.

FOURTH GROUP.

Hydro-mica and magnesian schists.-Soft and obscure, becoming quartzose and also hæmatitic, also with numerous beds of diorite. In Minnesota this is the iron-bearing horizon at Vermilion lake. It is Nos. vI to XIV at Marquette, Nos. IV to vi at Penokee, and Nos. vi to xi at Menominee. Maximum thickness 4450 feet.

FIFTH GROUP.

This is the group of gray quartzite and marble. It is represented by No. v at Marquette, Nos. II to v at Menominee and Nos. I to III at Penokee. In Minnesota this horizon seems to run along the south side of Ogishke Muncie lake, near the international boundary, and perhaps includes the great slate-conglomerate which is there represented. Normal thickness from 400 to 1000 feet; but if the great conglomerate of Ogishke Muncie be included here, the thickness of this group in northern Minnesota will exceed 6000 feet.

SIXTH GROUP.

Granite and syenite with hornblendic schists.-This lowest recognized horizon has frequently been styled Laurentian. In Minnesota it, is found on the international boundary at Saganaga lake, and large boulders from it are included in the overlying conglomerate at Ogishke Muncie lake, showing an important break in the stratigraphy. Thickness unknown but very great.

These six great groups compose, so far as can be stated now, the crystalline rocks of the Northwest. Their geographic relations to the non-crystalline rocks, if not their stratigraphic, have been so well ascertained, that it can be stated confidently that they are all older than the Cupriferous series of lake Superior, and hence do not consist of nor include metamorphosed sediments of Silurian or any later age.1

This statement of the grand grouping of the crystalline terranes of the Northwest may be varied by the addition of detailed and minor distinctions and by subdivisions, but its correctness rests upon careful observations and reports of competent geologists, and cannot at present be gainsaid.

Examining these groups more closely we find:

I. We have, beneath the red tilted shales and sandstones, a great granite and gabbro group. This has been variously regarded by different geologists. While by many early observers it was classed as older than the series which has latterly been designated Huronian, and by others styled igneous and local, it has, by Brooks, been placed with that series and denominated "the youngest" of the Huronian strata, though no such rocks had ever before been mentioned as pertaining to the Huronian. By Irving it has been made the base of his Kewenawan. By Hunt it has been parallelized with the Montalban. It includes, in my opinion, the felsites and porphyries which have been styled Arvonian, and it is very certain that in many places it has passed for typical Laurentian. The gabbro is very generally admitted to be of eruptive origin, and in its great development in Canada it was once styled Upper Laurentian, and later was known as Norian. While the gabbro is certainly eruptive, the associated granite and gneiss exhibit evidences of being metamorphic in their nature. In northern Minnesota this horizon of granite is characterized by a red color and it has an aggregate chemical composition almost identical with that of some of the associated felsites. The magnetite of the gabbro is often highly titaniferous and so abundant that the rock has attracted attention as an iron ore. The gabbro does not always appear where the granite is present, but extensive areas of granite are spread out without any sign of variation, interruption or alternation with the gabbro. In other places these

1 The term Silurian here is understood to cover nothing below the base of the Trenton.

two rocks are intricately and intimately mingled both horizontally and perpendicularly; but the gabbro may be considered in general as the underlying formation. Both these rocks seem to have been molten, and simultaneously so, in some places; but in the great mass of the red, granitic rock, there is a gneissic structure, and in its finely crystalline state, when it seems to vary to felsite, it exhibits a laminated structure which is evidently due originally to sedimentation. Along these laminations, and coincident with them, is a finely lined striation which exhibits the "streamed" structure, sometimes appealed to, to show the igneous nature and origin of the rock. These felsites are occasionally arenaceous, with irregularly rounded or sub-angular quartz grains, and sometimes are porphyritic with quartz and orthoclase. Veins of red granite intersect the gabbro, and the gabbro surrounds isolated masses of the granite. Transported, boulder-like masses of both are found embraced in a common paste among the later igneous outflows of the Cupriferous, where their existence is as great a puzzle as that of pebbles of red felsite and quartz-porphyry in the red conglomerates. This red granite, so far as I have observed, generally consists largely of orthoclase, and in several instances passes imperceptibly into red felsite. It contains also quartz and hornblende, the latter generally changed by decay. The gabbro, when unaffected by proximity to the red rock, consists of the three essential ingredients: labradorite, diallage and magnetite, with some necessary products of alteration, but in the vicinity of contact with the red rock it also holds orthoclase and quartz.

II. Below this granite and gabbro group is a series of strata that may be designated by the general term mica schist group. This is the principal, but not the only, horizon in which mica schist exists. This division is penetrated by veins and masses of red biotite-granite which appear to be intrusive in somewhat the same manner as the red granite in the gabbro overlying. However, whether this granite is exotic, or can be referred to aqueoigneous fusion and transmission of the sedimentaries in a plastic state through fissures in the adjacent formations, is a question which still is a matter of earnest investigation. The existence of the great associated igneous gabbro is suggestive, if not demonstrative, of the presence of an adequate agent for such a metamorphismunless it be claimed, indeed, that such an extravasation of molten rock could take place without any marked and traceable effect on