TABLE VI.

Reduction of Vanadic Acid by means of the Rotating Cylinder of Zinc and Correction by Silver Sulphate.

taken as ammonium

per minute

acid may be reduced by the rotating zinc reductor. Experiments with an amalgamated zinc cylinder showed that in this case the reduction proceeds similarly but much more slowly.

It has been shown that while vanadic acid may be reduced within a reasonable time, in presence of dilute sulphuric acid, by the action of massive silver of considerable area in the boiling solution, the reduction may be made very rapid with the aid of rapidly rotating anodes of silver or copper in the electrolytic cell, or by a rapidly rotating reducer of zine, over-reduction being corrected by the action of silver sulphate.

ART. XV.-Notes on the Geology of Rhode Island; by A. C. HAWKINS.

INTRODUCTION.

The recent paper by B. K. Emerson,' covering the geology of Massachusetts and Rhode Island, with its geologic map, sets forth in a somewhat generalized way that of the area included in western Rhode Island. The material herewith submitted, largely the results of investigations in the latter area by the present writer during the years 1912 1916, is in accord in many important respects with Professor Emerson's conclusions, and may serve to furnish certain of the geologic details not hitherto available.

ACID IGNEOUS ROCKS.

The State of Rhode Island is underlain by a great granitic batholith, whose members, now all more or less badly sheared, are represented by three principal types, the Milford, Northbridge, and Sterling granite gneisses. These are biotite granite gneisses of closely similar composition, each with its distinguishing characteristics (although certain phases of each resemble the others very closely at times), and together or separately they have invaded all of the older rocks of this region. The part of the granitic mass exposed to the west of the Carboniferous sediments of the Narragansett Basin is in its arrangement about as follows: In the northwestern portion of the State, as far south as the vicinity of Moosup Valley, and from thence southeastward toward East Greenwich, is found the Northbridge granite gneiss, covering the western half of Providence County and a portion of Kent County; northwest and west of Providence, in the eastern half of Providence County, as far west as the general vicinity of Woonsocket, and perhaps somewhat farther westward (compare Emerson, op. cit.), occurs the Milford granite gneiss; and farther south, from East Greenwich southward toward Point Judith and 1. S Geol Survey, Bull 597, 1917.

* Emerson, B K., and Perry, J. H., U. S. Geol. Surv., Bull 311, 9 10, 45 47, 17. Longlin, G. F., this Journal, 29, 447 457, 1916; U. S. Geol. Sarv, Bull. 492, ·S, 1912.

westward to the Connecticut line, the Sterling granite gneiss is to be seen. Their age relationships are somewhat obscure, since contact zones are usually either covered with glacial debris or occupied by streams or tidal estuaries. Some evidence as to age is, however, available.

Much of the contact zone between the Northbridge and Milford granite gneisses is occupied by an acidic granite phase like the "northfieldite" of Emerson. Yet a mile northwest of Oak Valley, R. I., near the road, at a point where it makes a sudden detour, just southwest of the right-angled cross-roads, a granite gneiss of Milford type is in contact with a very dark phase of the Northbridge granite gneiss. The former appears to intrude the latter and to include portions of it. From these rather unsatisfactory bits of evidence it has been concluded that the Milford is probably at least slightly younger in age than the Northbridge. Between the Northbridge and the Sterling, aplites are found to obscure the relations in the principal places where exposures are available. On Nooseneck Hill, and again just north of Summit, however, a coarse porphyritic pink granite gneiss is found to be thoroughly interlaminated with a fine-grained gray granite gneiss type. This is interpreted to represent the contact zone between the Northbridge and the Sterling and to indicate that the contact is gradational. The Northbridge and Sterling granite gneisses are alike in many respects, and it might be reasonable even to suggest that the Northbridge type may be a modification of that of the Sterling, due to excessive assimilation of basic material (hereafter to be more fully discussed), perhaps in conjunction with processes of differentiation which were possibly set in motion by the latter process. It is to be hoped that in spite of scarcity of good exposures the relations of these granite gneisses may in time be discovered in detail. (See further discussion below.)

East of the Carboniferous sediments of the Narragansett Basin a similar series of granite gneisses is exposed. Exact relationships with the foregoing types are not known, but it has been suggested that certain of the rocks closely resemble the Milford and Northbridge type; and

* Emerson, B K., this Journal, 40, 212 217, 1915,

Warren, C. H., and Powers, S, Bill. Geol. So». Am., 25, 459, 1914. (See also Emerson, op, eit i

R

Emerson and Perry, op. cit. 10. b Shaler, N. S. Woodworth, J. B., and Foerste, A. F., U. S. Geol. Sirv., Mon. XXXIII, 274, 1899.

while there is little or no evidence of the relative age of this granite gneiss series where it is adjacent to the green schists of Little Compton, R. I., it is the writer's opinion that they do intrude and include these schists.

The pre-Carboniferous age of the Northbridge granite gneiss is established beyond, doubt by its relations to the Carboniferous sediments in the area south of Woonsocket. From the latter city, a long narrow area of siliceous sedimentary rocks, the southward extension of the "Bellingham Series" of Warren and Powers, called by the present writer the Woonsocket Basin Series, mentioned and outlined in the Report of the Natural Resources Survey of Rhode Island for 1909, and so named, extends from the Massachusetts line for a distance of thirteen miles into Rhode Island. These sedimentary rocks resemble the Carboniferous sediments of the Narragansett basin (compare analyses in Table I), although they are much more thoroughly metamorphosed than the latter, being in places changed into biotite schists; but their age is established by a few fragmentary imprints of stems of Cordaites, obtained by the writer from a railroad cut just west of Woonsocket and northeast of Woonsocket Hill. The structure of the granite gneiss bordering this basin, as indicated by the dip and strike of its foliation, shows that the Woonsocket Basin sediments occupy the eroded summit of an anticline with slightly northward pitching axis in the Northbridge and Milford granite gneisses (see map, fig. 1). For a distance of several miles south of Woonsocket the border of the sediments is indistinguishable from the granite gneiss of the side of the basin, the former grading into the latter through arkosie beds (which are best shown on the west side of the basin just north of West Greenville, R. I.). South of the Waterman Reservoir, north of North Scituate, is exposed a great thickness of Carboniferous sedimentary arkoses, basal beds of the series, containing large amounts of blue quartz grains which are especially typical of the Milford granite gneiss and at times are also present in the Northbridge, The Carboniferous sediments have been removed by erosion south of North Scituate, although the structural basin persists to South Scituate, where on account of the pitch of the anticlinal axis, it disappears. Warren, ar 1 Powers, op. cit., 44× 449.