so radical a change in chemical composition. These inclusions. must, by this theory, be considered to be fragments of older rocks, formed in this way. Still basic inclusions may be supposed to have been formed by mechanical agglomeration, and no doubt this has often been the case. But, in opposition to both these theories, it is in many cases evident that the inclusions were soft, and then the simplest view is that they were drops, or portions, of a partial magma, which at the temperature, existing immediately before crystallization, could no longer be held in solution by the principal magma, but separated out,

The great petrographical province of Iceland is characterized principally by enormous eruptions of plagioclase-basalts and exceedingly subordinate eruptions of rhyolites, which, however, are very numerous. No other eruptive rocks are known from Iceland up to this time.' If we considered the differentiation of the primary magma, which here was very basic, as a diffusionphenomenon, according to "Soret's principle," it would be incomprehensible why the differentiation never stopped with the production of an intermediate magma, and, moreover, this theory would demand that every little rhyolite-magma previous to the eruptions would have been surrounded by a broad zone, showing all transitions to the basaltic magma. In both cases these intermediate magmas should have been erupted at some time, but, as already mentioned, we know a hundred eruptions of rhyolite but not a single one of andesitic rocks. It therefore seems more probable that these intermediate magmas never existed in the petrographical province of Iceland, but that the acid partial magmas were separated out directly from the basic original magma, which by lowering temperature lost its homogeneity. The conditions of temperature and pressure being different in different places these acid partial magmas also became somewhat different, but may all be classified as soda-rhyolites. chemical compounds, which constitute the silicate magmasand which are not necessarily identical with the rock-forming

The

1 Refer to H. BÄCKSTRÖM: "Beiträge zur Kenntniss der isländischen Liparite" in Geol. Fören. Förh. 13, 667. (Stockholm, 1891).

minerals

are naturally more than two, and therefore the liquation must become very complicated, being not only a function of temperature but also dependent on the original proportions. Therefore, in other places, where the original magma had another composition, relatively stable andesitic magmas might be formed, but this was evidently not the case in Iceland.

Liquation is no doubt also a function of the pressure, but experimental data are wanting. Still it may be considered as probable that, if liquation would augment the volume of the magma, then pressure would act the same as increase in temperature, and inversely. The first is most frequently the case with liquid-mixtures.

The purpose of this communication is to give to liquation and not to diffusion its place as the working hypothesis, upon which the theory of differentiation is to be constructed. How far this theory may differ from the approximation to it, given by Rosenbusch in his "Kern"-theory, the future will show.

In conclusion, I wish to express my best thanks to my friend and colleague Dr. S. Arrhenius for much valuable information furnished me in numerous discussions on this and other subjects which lie on the border between petrology and physical chemistry.

HELGE BÄCKSTRÖM.

THE GEOLOGICAL STRUCTURE OF THE HOUSATONIC VALLEY LYING EAST OF MOUNT WASHINGTON.1

(With Plates V, VI, VII.)

Published with the permission of the Director of the United States
Geological Survey.

CONTENTS.

Introduction.

The area studied.

Views of Percival and Dana regarding the area.

Lithological characters of the horizons.

Explanation of map.

Geological structure of the area.

Structural features as shown in longitudinal sections.
Structural features as shown in transverse sections.
Structure of Tom's Hill.

The great Housatonic Fault.

Metamorphism along the fault.

Thickness of the Egremont Limestone.
Conclusion.

2

IN a former paper I have discussed the geological structure of Mount Washington and shown that in that mass we have to deal with a conformable series of beds embracing four distinct lithological members. These members are: (1) a lower dolomitic limestone-the Canaan Dolomite; (2) a lower schist member containing usually abundant garnets and frequently also staurolite the Riga Schist; (3) a calcareous member, in the valley a marble but on the summit plain of the mountain and along its base very micaceous and graphitic-the Egremont Limestone; and (4) a schist member very feldspathic and

1 Part of a report of work done as Assistant Geologist in the Archean Division of the U. S. Geological Survey, under the direction of Professor Raphael Pumpelly.

2 On the Geological Structure of the Mount Washington Mass in the Taconic Range. Journal of Geology, Vol. I., p. 717.

usually either chloritic or sericitic, but always free from garnets and staurolite-the Everett Schist.

The area studied. To the eastward of Mt. Washington, at a distance of five or six miles, flows the Housatonic river, its general course being like the crest-line of the mountain, nearly south. To the northeastward of the mountain the intervening area is a nearly level plain in which are extensive outcrops of the Egremont Limestone, sometimes with thin intercalated micaceous or quartzitic layers. This limestone belt extends almost to the river at Great Barrington and Sheffield Plain. South of the village of Sheffield, however, the level expanse of the plain is broken by the occurrence along its eastern margin of low, sharp ridges trending northeasterly to northwesterly, and increasing in number as well as in height and breadth in going south. The area covered by these ridges begins at Sheffield where two narrow ridges are separated by only a few hundred feet, and broadens steadily in going southward, thus narrowing the belt of limestone on its western border, and finally cutting it off near the village of Salisbury by making connection with the southeastern base of Mt. Washington. (Cf. Plate III. of Mt. Washington paper). Corresponding with the increase in breadth which characterizes the area in its southern portion, there is a marked increase both in the height and the width of the individual ridges. East of the Twin Lakes in Salisbury is Tom's Hill, which rises to a height of over 1,200 feet, while further south, to the east of the village of Salisbury, is Barack M'Teth (1,300 feet), and Watawanchu Mountain (1,300 feet), and farther east in about the latitude of Watawanchu Mountain is Mt. Prospect (1,460). This tongue of alternating schist ridges so sharply outlined, presents so much of unity in topographical and geological features as to be eminently suited to separate treatAs the ridges are composed of the Riga and Everett Schists, the area is closely connected geologically with Mt. Washington. This paper is devoted to the consideration of

ment.

To be distinguished from one of the northwest peaks of Mt. Washington which bears the same name.

the structure within this tongue-like area, which includes between twenty and twenty-five square miles. The field work was mainly done in 1888, though the southern portion of the area was revisited in 1891, when the writer was assisted by Mr. Louis Kahlenberg, and again in 1892 when he was assisted by Mr. H. J. Harris. The work has been in charge of Professor Pumpelly, then the head of the Archean Division of the U. S. Geological Survey.

Views of Percival and Dana regarding the area.-Though the map accompanying Percival's report does not indicate the schist areas within the area which is under consideration, he several times mentions them in the text. One is surprised to find how accurate were his observations and how correct his views regarding the area, notwithstanding the limited facilities and unsatisfactory condition of his survey. The following extracts from his report contain the more important statements which he made having reference to this area.

"It (the limestone) is accompanied throughout with Mica Slate sometimes forming thin interposed beds, and at other times extensive ranges. The Mica Slate, in the vicinity of the limestone, particularly when interposed in thin layers in the beds of the latter, is very generally dark and plumbaginous, but occasionally light gray, as in the more extended ranges. These latter usually occupy high narrow abrupt ridges, sometimes quite isolated, and at other times in longer ranges, generally with an irregular outline." (Pp. 126-127).

"A coarse dark Mica Slate, veined or knotted with quartz, and often abounding in staurotides and garnets, occurs especially in the north part of the ridge bounding, on the west, the valley south of Lime Rock village,

"The general surface of the valley, in the north part of Salisbury, in Canaan, and in the adjoining part of Massachusetts, is low and level, but traversed by ridges of Mica Slate, often high and abrupt, either isolated, or in long continuous ranges, the latter generally presenting a distinctly curved outline." (P. 129).

"Between these two branches extends a series of Mica Slate ridges, continued north from the ridge bounding the valley at Weed's Quarry (K1.) on

I

Report on the Geology of Connecticut, by JAMES G. PERCIVAL, New Haven, 1842, pp. 124-130.

2 Of the Housatonic Valley.