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West Rock

Silica...

Stoeckel's farm) was selected for analysis. It was crystallinegranular in texture; and it would be hard to detect by the eye any difference between it and many kinds of doleryte which are found in this region. The analysis shows that in chemical composition also it is very nearly the same. An analysis of a specimen of true igneous doleryte, from the trap ridge called West Rock, in New Haven, is placed beside it for comparison. METADOLERYTE, FROM STECKEL'S FARM.

Doleryte, from II.

Mean. 50:40 50:32 50.30

51.78 Alumina ....... 14 43 1471

14:57

14.20 Ferric oxide .... 2:48 2:47 2:48

3.59 Ferrous oxide ... 8.28 8:35 8:31

8.25 Manganous oxide

.46

.44 Lime ......

11:15
11:11 11:13

10.70 Magnesia ... 7.65 7.59 7.62

7.63 Soda . 3:01 3:08 3.04

2:14 Potash •43 •44 44

•39 Titanic acid..

1074

1.70 Chromic oxide...

tr. P,Q,.... 14 Ignition .........

.83 •78

.63

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3.04

100.65 101.13 100.89

99.89 Specific gravity --

3.03 The close resemblance between the igneous and the metamorphic rock will be noticed ; they differ from one another less than do the different varieties of doleryte. Moreover, observations made upon thin sections indicate that the rock is composed of pyroxene, a triclinic feldspar, and a black opaque mineral which the analysis shows to be titanic iron. The pyroxene is a dark-green variety, but clear and undecomposed. If we assume that the pyroxene of this rock is of the same composition as that of the New Haven dolerytes,* the magnesia indicates that it contains 55 per cent of this ingredi. ent, which being subtracted along with 3 per cent of titanic iron, leaves 41 per cent of a mineral, the oxygen ratio of which is very near to 1:3:6—proof that the feldspar is labradorite. Hence, the physical appearance, the chemical composition, and the proportion between the mineral constituents all show a very close resemblance to doleryte. The name of metadoleryte seems therefore to be particularly appropriate for this rock.

2. Metadiabase - The chloritic variety, which has been referred to, resembles diabase in appearance as closely as the preceding kind does doleryte. There are, however, wider limits of variation in texture and in the proportion between the mineral constituents than is noticed in diabase; for the rock is sometimes uniformly crystalline, and sometimes coarsely

* See this Journal, III, is, page 187.

I.

7041

24

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porphyritic. As would be supposed, there are no amygda-
loidal cavities or geodes either in the mass or in microscopic
sections. The analysis was made upon a specimen collected at
the Derby railroad cut where there is a fine display of these
rocks. The specimen was uniform in texture and of a light-
green color.
METADIABASE, FROM THE DERBY RAILROAD CUT.

Mean.
Silica

- 48.25 48.15 48.20
Alumina ...... 14.22 14:01 14:12
Ferric oxide ...

1.95
2:05

2.00
Ferrous oxide

7.39

7.43
Manganous oxide

1:30
1.19

1.24
Lime ........

11:53 11:47 11:50
Magnesia

8.26
8:11

8.19
Soda .....

2.63
2:56

2.60
Potash

23
Titanic acid

1.61 155
Water ....
2:11 2.29

2.20

-- 99.49 99.04 99.27 Specific gravity-........................3:02 The analysis, taken with the observations inade upon thin sections, shows that the rock is a mixture of pyroxene, chlorite, labradorite, and titanic iron, which are the constituents of diabase ; and hence this metamorphic rock is appropriately distinguished by the name metadiabase. The absence of carbonate of lime is noticeable, showing that in this case the ohlorite was formed simultaneously with the pyroxene, and not at the expense of the pyroxene, as in the case of the diabase of the trap dikes of the Connecticut valley, which always contains carbonate of lime as one result of the change. This rock in places contains pyrite, which is aiso frequent in trap. PORPHYRITIC METADIABASE ; South of MALTBY PARK.

II.

Mean. Silica.--. .--.-48:57 48.65 48.61

49.28 Alumina ........17.78 17.85 17.81

15.92 Ferric oxide...... •35 •16

25

1.91 Ferrous oxide .--. 8:44 8.48 8.46

10.20 Manganous oxide. -20

•20
20

37 ........11.17 11.14 11:16

7.44 Magnesia ....

7.74
7076

5.99 Soda ...... 2.73 2.82 2.77

3:40 Potash .... . .47

47

•72 Titanic acid..... 1:35 1:35

1:35

CO,

1:14 Water........... 1.60 1.65 1.63

3.90

1.

Diabase of Salton.

stall Ridge,

Lime ---

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100·44 Specific gravity-...

100:51 100*47

........ 3:01

100.27

2.86

are quitehic, and thes, but in those

There are varieties of this rock intermediate between these two, some specimens of wbich are beautifully porphyritic. In some kinds the feldspar is free from impurities; but in those varieties which are very feldspathic, and the feldspar crystals largest, these crystals are quite impure from the envelopment of chlorite. The porphyritic rock, from an outcrop near the Orange road, just south of Maltby Park, containing clear crystals of feldspar, was analyzed, and the result is given on the preceding page. An analysis of the diabase of Saltonstall Lake, from my former paper, is added for comparison.

This porphyritic ro :k is composed of the same minerals as the more compact varieties, for all of the ingredients can be easily recognized under the microscope. The possible presence of anorthite in the rock is suggested by the following analysis of some large grains of feldspar taken from an adjoining rock: Sio, 45:52, Al,0, 29-84, MgO 2:35, CaO 15.99, Nao 1.61, KO :37, ignition 2:38 = 98.06. This analysis was made by Mr. E. S. Dana some years since, but he states that the microscopic examination, and the analysis itself, show that the grains were very impure crystals of a triclinic feldspar, and as all the calculations upon the analyses point to the presence of labra. dorite, we cannot assume that any of the rocks wbich have been analyzed contain anorthite, though it is very likely to exist in the rocks of the series, since a constant composition in the feldspar could not be expected in the different layers of a rock made up of shifting sediments.

3. Metamelaphyre-a specimen taken from an outcrop on Stockel's farm is so fine grained as to appear nearly cryptocrystalline; it is broken into angular fragments like some of our trap rocks, and in fact resembles some compact trap so closely as to make it impossible to distinguish it by the eye alone. Its analysis afforded the following results:

METAMELAPHYRE, FROM STECKEL'S FARM.
1.

II.
Silica ...........55.03

55:10

55:07 Alumina ........14:38

13.98

14:18 Ferric oxide ..... 7.15

7.25

7.20 Ferrous oxide .... 1.85

1.99

1.92 Manganous oxide. •30

.30

•30 Lime ......... 9:05

9:01

9:03 Magnesia ........ 6:02

5.94

5.98 Soda......

4.08
4:14

4:11
Potash ....
•38

37 Titanic acid.....

1:56

1:56 Water ..........

•75

.72

Mean.

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100.39

100•48 Specific gravity ............

10044

2.99

If we assume that the pyroxene of this rock bas the same composition as that of No. 1, we calculate, from the magnesia that it contains, 44 per cent of this ingredient; then, deducting three per cent of titanic iron, we have left a remainder of 53 per cent, wbich has very exactly the ratio and composition of oligoclase. This mineral constitution appears to be justified by the microscopic examination, since no free quartz or other mineral can be detected. If we restrict the use of the term melaphyre, as it is done in some recent works on lithology, to a mixture of oligoclase and pyroxene, with some titanic iron, the rock here analyzed is melaphyre in composition as well as appearance; and being a metamorphic rock, it is metamelaphyre.

We thus have representatives of the larger part of the pyroxenic igneous rocks, in positions which show conclusively that they are of metamorphic origin. The fact that metamorphic action can produce rocks exactly like the igneous in external aspect and chemical constituents is of great interest in the study of rocks.

ART. XIV.- On a new Tertiary Lake Basin ; by GEORGE B.

GRINNELL and EDWARD S. DANA.

SEVERAL Lake Basins of Ter iary age have already been discovered in the Rocky Mountain region, and the more important of them have been carefully explored. Those of Eocene age have only been known since 1870, but the Miocene deposits of the White River have long been noted for their wonderful scenery, as well as for the number and variety of the mammalian remains found in them. Another Miocene basin is known in Oregon, and both the lake beds of this period are overlaid by deposits of Pliocene age.*

During the explorations carried on last summer under the direction of Col. Wm. Ludlow, Corps of Engineers, a series of Tertiary deposits were identified by the writers near Camp Baker, Montana. These deposits indicate the existence in this region of a Miocene lake basin, which was succeeded by another lake basin in Pliocene time. As these basins are quite distinct from those heretofore known, it is considered important to put the fact of their discovery on record.

Camp Baker is situated on Deep Creek, a stream which flows into the Missouri River above Sun River. It lies about fifty miles nearly due east of Helena. It is surrounded on all sides by mountains, of which the Big Belt Range, lying immediately to the south or southwest, is the highest and most conspicuous.

* This Journal, III, vol. ix, p. 49, Jan., 1875.

The Little Belt Mountains lie to the north, and the Crazy Woman Mountains to the southeast, though at a greater distance.

The Tertiary beds found here consist for the most part of homogeneous cream-colored clays so hard as to be with difficulty cut with a knife. The beds are horizontal and rest un. conformably upon the upturned yellow and red slates below. The clavs of which they are formed resemble closely those found in the Miocene beds at Scott's Bluffs near the North Platte River in Wyoming. The deposits at Camp Baker lave been extensively denuded and now here reach any very great thickness. At a point about three miles southeast of the Post, some bluffs were noticed where the Miocene beds attained a thickness of 200 feet, and these were capped by fifty feet of Pliocene clays, both beds containing characteristic fossils. In the underlying Miocene beds were found a species of Rhinoceros, several species of Oreodon Leidy and Eporeodon Marsh, a canine tooth apparently of Elotherium Pomel, and remains of Turtles. In the Pliocene beds the principal fossils were a species apparently of Merychyus Leidy, remains of an equine smaller than the modern horse, and Pliocene Turtles. These fossils have not yet been carefully studied, and for this reason their relations to the remains found in the other lake basins of similar age cannot here be stated.

We saw the first exposures of these beds a few miles west of the Sulphur Springs, just after crossing a rather high ridge of trachyte through which Deep Creek flows in a narrow and picturesque cañon. This point is about six miles southeast of Camp Baker. From here the lake bed was traced continuously along Deep Creek for a distance of fifteen miles, extending quite up to the mountains on the eastern side at least. Beds of the same character, containing similar fossils, were found on White Tailed Deer Creek, a branch of Deep Creek, about seven miles to the north of Camp Baker, as well as on Camas Creek to the south west of the Post. Traces of this deposit, containing what appear to be remains of Rhinoceros, were also found two miles or more south of Moss Agate Springs, and at a considerable elevation above the creek bed. With more time than we had at command they could no doubt have been traced much farther, although in many places the beds have been washed out, or have been covered by the later local drift.

These Tertiary beds were all laid down after the elevation of the mountains and the igneous eruptions. They are, as has been said, perfectly horizontal, and are often seen covering over ridges of trachyte. The line of separation between the Miocene and Pliocene beds is in some places well marked. It consists of about six feet of hard sands, interstratified with

This Deep

along Baker. Sen

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