REVIEWS.

Eruptive Rocks from Montana. By WALDEMAR LINDGREN. Proc. Cal. Acad. Sci. Ser. 2, Vol. 3. 1890.

A Sodalite-Syenite and other Rocks from Montana. By W. Lindgren, with analyses by W. H. MElville. Am. Jour. Sci. Vol. 45. April 1893.

Acmite- Trachyte from the Crazy Mountains, Montana. By J. E. WOLFF and R. S. TARR, Bull. Mus. Camp. Zoology, Harvard College. Vol. 16, No. 12. (Geological Series, Vol. 2).

Contributions to our knowledge of the mineral and chemical composition as well as the relationships of the igneous rocks of particular regions, however fragmentary, are of the greatest importance; especially when they relate to the vast areas of North America which remain almost unknown to the petrologist. The exploration of the great belt of country, one hundred miles wide, extending from California to Colorado and Wyoming along the fortieth parallel of latitude, by the geologists under Mr. Clarence King, constitutes the one great systematic study of the volcanic rocks of any considerable area on this continent. Less extensive investigations of smaller areas, isolated from one another and often separated by long distances, have been made from time to time, and to some extent have been published. But a large part of the work already done has not yet been printed. The facts so far brought to light show that the rocks of the Great Basin and the Pacific coast differ as a whole from those occurring in the eastern portion of the Rocky mountains and the region immediately east of it. This difference consists mainly in the greater abundance of the alkali-bearing rock-making minerals in the rocks of the latter region, caused by the relatively higher percentage of sodium or potassium in the magmas from which they have been derived.'

The recent papers by Mr. Lindgren and by Messrs. Wolff and Tarr illustrate this characteristic of the volcanic rocks of Montana along the frontal ranges of the Rocky mountains. All of the rocks described occur as intrusive bodies; laccolites, sheets, dikes or necks. They

1J. P. IDDINGS: The Origin of Igneous Rocks. Bull. Phil. Soc. Washington, Vol. 12, pp. 138, 139, 184.

were erupted in early Tertiary or late Cretaceous time in most cases, but their exact date is not known. Owing to extensive erosion the extrusive forms of these rocks, if they ever reached the surface, have been entirely removed.

Mr. Lindgren observes in the first paper cited that the rocks of this region appear to be more varied in chemical composition than the series usually found in the Great Basin; magmas rich in potassium are frequent, crystallizing as trachytes; often they are very basic, and contain much sodium, resulting in the abundant separation of such minerals as nepheline, sodalite and analcite.

The more or less acid rocks in the Little Belt mountains and at various points in front of the main range, west of Fort Benton, constitute dacites, hornblende-andesites, and diorites. Similar rocks also occur in the Moccasin mountains. They vary much in structure and composition, and form a natural group. The prevalent habit is porphyritic, but there appears to be a continuous series of transitions from porphyritic to fine granular rocks. The phenocrysts are feldspar and hornblende, and sometimes quartz and mica. The porphyritical feldspars are in part orthoclase in varying quantities, and there is reason to believe that these rocks pass by gradual transitions into trachytic and rhyolitic forms.

Those varieties free from phenocrysts of orthoclase and quartz grade into medium grained diorite, analogous to Stelzner's “Andendiorit," which contain besides plagioclase, hornblende and biotite, a little orthoclase and quartz as the last minerals to crystallize.

Of the more basic rocks, a part are syenites and trachytes, and a part basalts. The syenites which form dikes consist principally of orthoclase, plagioclase, biotite and a pyroxene, probably malacolite. They are called augite-syenites. The syenite from near Dry Fork, Little Belt mountains, contains, in addition to these minerals, allotriomorphic grains of an isotropic substance, probably sodalite. The rock contains 5.50 per cent. of K 20, and 4.14 per cent. of Na2O. The augitesyenite from the Highwood mountains is coarsely granular, and contains 5.66 per cent. of K20 and 7.88 per cent. of Na2O. This syenite is surrounded by trachytic and basaltic dikes; and in one case a dike of syenite was seen cutting one of the basaltic dikes.

The syenite from Square Butte at the northern end of the Highwood mountain is characterized by a noticeable percentage of sodalite and analcite, and has been called sodalite-syenite. Its chief constit

uents are orthoclase, albite and hornblende. The relative proportions of the minerals has been estimated to be: orthoclase, 0.50; albite, 0.16; hornblende, 0.23; sodalite, 0.08; analcite, 0.03. The hornblende was analyzed and found to correspond to barkevikite. Mr. Lindgren calls attention to the resemblance in chemical composition between this rock and many nepheline-syenites, except for the relatively higher percentage of K20 in the rock from Square Butte. He also notices the striking similarity between the analysis of this rock and those of certain leucitophyres from Rocca Monfina, and remarks that under different conditions the same magma, now crystallizing as a sodalite-syenite, might have produced a leucite-feldspar rock.

Trachytic rocks, with a great variety of habits, are abundant in the Highwood mountains. The essential minerals are sanidine and augite, with less prominent biotite. The augite is deep green, often somewhat pleochroic, and evidently contains an admixture of the ægirine molecule. It is very characteristic not only of the trachytes but also of the basaltic dike rocks of this region. These rocks form a connected series, the members of which differ in the relative quantities of augite and sanidine composing them. At one end of the series is a rock consisting almost wholly of feldspar, and at the other end a dark basaltic rock with porphyritical augites and a groundmass of sanidine and augite. In structure these rocks range from holocrystalline and granular to glassy. Some of the trachytes contain small crystals of sodalite (?) inclosed in sanidine. In one form of the rocks sanidine ceases to be the prominent phenocrysts and augite takes its place, and olivine occurs in the groundmass, which consists of feldspar and colorless glass easily soluble in HCl. Associated with the sodalite-syenite of Square Butte are dark colored basaltic rocks, which occur in three sheets at the base of the butte. Surrounding the butte there are numerous dikes apparently radiating from the central mass. One of these basaltic sheets contains phenocrysts of augite, olivine, brown mica, and white isometric crystals whose original character is uncertain. The rock is considerably decomposed. Another of the sheets is like analcite-basalt but is also decomposed. The third is coarsely granular and approaches theralite in composition.

The rocks described as analcite-basalts occur in dikes and possibly as necks in association with the rocks already described. They consist of augite, olivine, magnetite, and a mineral, which from its form and optical properties, and from its chemical composition appears to

be analcite.

Biotite is sometimes present in small quantities. From the very fresh appearance of these rocks it seems probable that the analcite is a primary crystallization from the molten magma. The groundmass of the rock consists of augite and small crystals of analcite with magnetite. Mr. Lindgren calls attention to the difficulty of distinguishing glass, if present, from isotropic analcite.

In the Bear Paw mountains there are dikes of rocks related to those just described and which correspond to the lamprophyres of Rosenbusch. They are dark, fine grained, and porphyritic with phenocrysts of augite and long flakes of brown mica. The groundmass consists mostly of lath-shaped plagioclase, augite and mica. Some varieties with phenocrysts of olivine and augite, in a glassy groundmass without feldspar, approach certain limburgites.

The paper by Messrs. Wolff and Tarr is confined to a description of certain trachytic and syenitic rocks in the Crazy mountains. The first notice of the interesting rocks of this locality was published by Mr. Wolff in 1885, and he has since undertaken a much more extensive investigation of the same group of rocks, which is not yet completed. The trachytes form dikes, sheets and laccolites in the northern portion of the range, and are associated with theralite. Like the theralites and some other rocks of this range, they are coarse grained, almost granitic when in thick sheets, fine grained and porphyritic in the smaller sheets, dikes, and apophyses. When occurring in the latter forms the rocks have a trachytic habit, and are called acmitetrachyte. The phenocrysts are glassy feldspar, augite and small sodalites. Biotite is scarce. The feldspar is soda-microcline or anorthoclase. The augite is pale green at the center, and becomes dark green at the margin, where the optical characters are those of aegirine, similar to that in the theralite. The groundmass consists essentially of lath-shaped feldspar and acicular crystals of aegirine. With the green aegirine a few brown needles of acmite occur. There is a variable amount of interstitial matter between the feldspars of the groundmass which is probably nepheline in part, and partly analcite, derived from the alteration of the nepheline.

The coarse grained forms of the rock, or syenite, consist of the same essential minerals as the trachytic varieties. Sodalite is rare in the coarse rocks, and acmite is not always present. Chemical analyses of these rocks are published, but the discussion of them is postponed until the monograph of the whole group of rocks is pre

pared. The resemblance between certain features of the rocks of Montana and those from Arkansas, described by J. Francis Williams, is pointed out by each of the writers cited. The resemblance to the lamprophyric rocks in the Absaroka range, Wyoming, east of the Yellowstone National Park, is also noticed.

Some of the petrographical characteristics of the rocks of this region are: The prevalence of orthoclase in many intermediate and basic rocks, leading to the frequent occurrence of trachyte and syenite and some forms of lamprophyre, as well as its presence in prominent crystals in the andesites and porphyrites, and the frequent occurrence of dark green augite and aegirine, and occasionally of acmite.

The difficulty of distinguishing colorless glass from isotropic analcite, both of which may occur in certain varieties of lamprophyre, makes it necessary to use the greatest care in determining the character of the apparent base in these forms of rocks. It seems probable to the reviewer that in some instances, in which an amorphous glass has been described as forming the matrix of the microscopic crystals in some lamprophyric dike rocks, it will be found that a definite. isotropic alkali mineral is present, and that the rock is holocrystalline JOSEPH P. IDDINGS.