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Thin section of Trap from Williamson's Point,

Lancaster Co. Pa.
Magnified 100 Diameters.

MIC

and as usual are characterized by their tendency to separate into two or more parts, colored respectively light brown and pale blue.

The four masses of pyroxene are equally distinguishable by the peculiar net work of clefts which cover surfaces, which in polarized light present usually one of the tints here distributed among them. It is often the case when a thin slide has been carefully and evenly made, that at a certain position of the analyzer all the labradorite divides itself into its two characteristic colors depending upon the positions of the optical axes of a pair or a series of the twins. With pyroxene it is different. The fragments, in these traps at least, rarely show definite crystalline form, and in any given position of the analyzer there may be found specimens exhibiting any of the indefinitely large number of gradations in color between bright green and dark violet which accompany the rotation of the analyzer through the angle which separates the projection of their optical axes.

Only a single hexagonal section is given in the picture, but these figures are distributed, though not profusely, throughout the mass. This is probably a minute column of Apatite, and the low percentage of Phosphoric Oxide in the accompanying analysis sufficiently explains why these forms are comparatively rare.

A number of these hexagonal forms having been examined, it was found that when most symmetrical they exercised no influence on polarized light and were, therefore, sections perpendicular to the optical axis of an hexagonal crystal since the basal plane assumed this form.

But where the hexagons were distorted, or in the frequent cases where they were covered by a film of vitreous pyroxene or labradorite, the extinctions were more or less irregular.

One of the quasi-hexagons measured 0.048 mm. between the parallel edges.

The comparatively large rhombic figure is in all probability a section of calcite parallel to one of the planes of the rhombohedron. Several of these figures were measured and examined. One of them was 0.11 millimeter in the longer axis. The angles as measured were 81° 03' and 98° 36' respectively. This crystal showed four positions of maximum transparency, and four positions of extinction alternating with each other at distances of 45°* Another and the largest similar section which was measured showed a longer axis equaled to 0.25 mm.

An examination under the improved Fuss's microscope with a magnifying power of 275 diameters gave:

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In this case there was no general extinction of light during one revolution with or without the quartz prism.

"The microgoniometer having been set at zero when the Nicols were crossed, the succeeding positions of extinction were very nearly 0°, 90' 180", and 270°

PROC. AMER. PHILOS. SOC. XVIII. 102. M. PRINTED JAN . 25, 1879.

The modifications of light at a few points seemed to be caused by impurities.

A small rhomb examined in the Fuss instrument, under a magnifying power of 275 diameters gave:

Acute angle (imperfect) 73° 30';

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A profusion of small black specks distinguished the face of this crystal which was apparently therefore not homogeneous. It showed four extinctions.

A third rhombus at 275 diameters showed:

Acute angle 72'

Obtuse angle 101

Neither of the above angles was perfect.
This crystal also showed four extinctions.

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The ground mass is composed of minute objects, often stellate in structure, among which are probably small columnar masses of Rutile, as the titanic oxide in the analysis suggests. They are exceedingly small and, except here and there, do not indicate any definite order or arrangement.

This ground mass is not certainly determined. The tufts are sometimes flat and frayed at the ends like a piece of worn cloth. They polarize feebly within different shades of brown.

The following is an analysis of this trap by Dr. Genth.

Silica (Si0.2) 50.79

Titanic Oxide (Ti02) 0.70

Phosphoric Oxide (P,05) 015

Alumina (A1,0i) 14.19

Iron Sesqui-oxide (Fe./)3) 3.84

Ferrous Oxide (FeO) 7.44

Manganous Oxide (MnO) 0.48

Lime (CaO) 9.75

Magnesia (MgO) 7.88

Potash (K20) 0.95

Soda (Na,0) 1.89

Ignition. 1.95

Total , 100.01

Herewith are presented, side by side, average analyses of Labradorite and of Pyroxene both calculated from the data given in the last edition of Dana's System of Mineralogy (Fifth Edition, 1877). The former is based upon forty analyses of Labradorite and the latter on eighteen analyses of Pyroxene from eruptive rocks.

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In the following table the percentage of each constituent in Dr. Genth's analysis of this trap is doubled, and the result compared with a column showing the sums of the percentages of the average Labradorite, and the average Pyroxene :

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It will be seen that the two columns agree remarkably well in most particulars, which is the sanie as saying that the composition of the rock is very nearly what the chemical analysis of a mixture of one molecule of Labradorite and one molecule of Pyroxene would show.

In Report of Progress C, 1876, a number of similar traps were discussed and analyses given : amongst others of one from “ West Rock,” Connecticut, and one from Beeler's farm, York County.

The composition of the former, like the one here considered, agreed more nearly with a distribution of Labradorite and Pyroxene molecules in the proportion of one to one, while the “Beeler'' trap corresponded more nearly with the mixture of two molecules of Labradorite with one of Pyroxene. * 0.70 Ti O, and 0.15 p. c. P, Og included in SiO2. + Ignition. Second Geol. Survey of Penna.

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