2. Observations on the crystalline form, optical characters, and cleavage of Hortonolite; by JOHN M. BLAKE.

The examination and measurement of crystals of Hortonolite, which were placed in my hands for this object by Prof. Brush, show unmistakably that the mineral belongs to the chrysolite group. A comparison was made with other members of the group, to determine its relation to them. For this purpose, several species were measured, crystals having been placed at my disposal by Prof. Brush. The points compared were the occurrence and proportional development of planes, and to some extent the optical properties and cleavages. This examination is not yet completed, but it being desired that a description of this mineral should be furnished as soon as possible, the results must be given in a form that will require the least explanation. For the present purpose, the symbols of Des Cloizeaux are adopted, so that comparison can be more readily made with the figures in his mineralogy. It will be noticed that the "vertical prism" is differently selected.

The observed planes on this variety, are: h', g', e1, a', g3, b and y; or in Dana's symbols: O, i-i, i-2, 1-ī, 1-ì, 1-2 and 1-4. A deposition of some foreign substance had destroyed the brilliancy of the planes, and this could not be entirely removed so that they would give perfect reflections; and, besides this, some parts of crystals appear to have been originally rounded. Fig. 1 is proportioned from some of the larger crystals.

They were partially imbedded, so that but a portion of the planes could be distinguished on any one of them. The intersections with the other planes satisfactorily determined the planes b, on these particular crystals.

Fig. 2 is a common form of the medium-sized crystals.

The upper planes on the front side, can be explained as the planes y. The directions of their intersections with e1 and an approximate measurement of their inclination on g1 leaving little doubt of their identity with this plane. The goniometer readings were as follows on this crystal.

For the prismatic zone: g', 0°; e', 65° 35'; e1, 116° 15′; g', 181° 20'; e1, 245° 50'; e1, 295° 15' ?, g1, 0° 45'.

For zone 2, including the front upper planes: g', 0°; 7, 78° to 80; 7, 100 to 101°; g', 178°.

Zone 3, including the back, upper planes: g', 0°; bì, 69°; a', 90'? b, 110°.

From the first zone the average inclination of g1 on e' is 65' (normals); Brooke and Miller give 65° 2. From the second zone we have y on g', 77° to 80°; B. and M. give 79° 40′. From zone 3: g' on bi, 69° to 70°. The same authority give 69° 57'. The following readings were taken from another crystal having the planes g3 better defined: g', 0°; g3, 320°; h1, 268° 20′; g3, 220° 30'; g' 179°. From this the average for g' on g3 is 40° 45'; B. and M. give 40° 27'.

An examination of the measurements above recorded will show that although the mean values deduced from them accord well with the angles of chrysolite, yet there is some variation when the angles are taken singly. This variation is not greater than the imperfections of the planes gave reason to expect.

It is probable that the planes 7, a', and b, occur together on some of the crystals. These planes lie very near together, and a slight rounding of the crystal might be expected to obliterate them. Fig. 3 represents an occasional form. It is introduced to show the variation in crystals upon the same specimen. Another small crystal had the prismatic planes nearly equal in breadth, and g3 largely developed, while the other terminal planes were rounded.

3.

Notwithstanding this great variation in development, the crystals do not at all resemble those of hyalosiderite in habit; neither do they resemble certain crystals occurring as furnace products, which I have directly compared with them.

The crystals appear black externally, but some small portions are found which are transparent in thin sections, and have a yellowish color. A portion was selected from a section parallel to g', which admitted of an approximate measurement of the separation of the optical axes. The acute bisectrix was found perpendicular to g', and the axes to lie in a plane parallel to h. Their separation was, from 83° to 86°, measured in olive

oil.

The smallness of the transparent part of the section and the presence of one or more oblique cracks in its interior, rendered it difficult to determine satisfactorily the direction of the dispersion of the axes, but it was judged that the separation of the red rays was greater than for the violet about the bisectrix normal to g'. This agrees with Des Cloizeaux's observations on the dispersion and plane of the axes in a specimen from Torre del Greco, and in tephroite. In the former he found the separation to be 86° to 87° about a bisectrix normal to p, and in tephroite the separation of the axes for the

red rays was 83° 41', and 80° 29' for the blue, measured in oil. The bisectrix in this case being negative, and normal to g'.

For comparison, a measurement was made of olivine occurring in lava from the Sandwich Islands, and the separation of the axes about the normal to g' was found 107°; in a furnace product 68°. Both were measured in olive oil. Reducing the measurement on the chrysolite, from Torre del Greco, to the same bisectrix, but without making a correction for the refraction of the oil, and of the mineral which would be necessary to make the results compare accurately with the others measured in oil, we have 94°.

It so far appears that the optical axes in this group lie in a plane parallel to h1, but vary in separation with the chemical composition, according to some law. It seems generally preferable to make sections parallel to g', for the purpose of comparing species; since this is usually a cleavage plane, and there is then a better chance of success in cutting some varieties which are brittle, and the plane p seldom occurs, while g1 is often well defined, and serves as a guide. It is to be understood that reference is made to such sections as can be most readily cut for the purpose of comparing varieties of this mineral, as is done with the micas.

The cleavage which occurs parallel to h', is perhaps most clearly defined. There are two cleavages about the plane g1, and possibly one not well defined parallel to this plane. They lie in the prismatic zone, g', e', etc. This was first discovered in the following way: a fragment showing several cleavage surfaces, and at least one plane of the crystal, was measured on a zone with the following result: first cleavage (goniometer reading) 0°; second cleavage, 49° 30'; a plane of crystal (g'), 203° 30'. On plotting the normals, it was found that the normal of the plane of the crystal, when extended, bisected the angle of the normals of the two cleavages. Hence the cleavages are symmetrically situated about the plane g', and in the zone, g',e', et, etc.; since another cleavage was found on this fragment perpendicular to the intersections of the two others, giving as an approximate measurement an inclination of 92° 20' on one of them, and this cleavage had also a different appearance to the eye, and was therefore considered to be the cleavage parallel to the basal plane h'. No reflection from a cleavage was detected on this specimen parallel to g1. The three cleavages actually found gave bright reflections in a good light, but admitted of no very exact measurement; though sufficiently exact to show that those on the prism are not parallel to any observed plane, except it be the rarely occurring planes el or e. The inclination of el to g', is given as 28

ed: 13', and of e, 23° 13', (normals). The cleavage as above deterto mined measures 24° 40' on g'. This result appeared so extraordinary, that a piece was clipped out of the plane g', on a crystal upon which there could be no mistake as to the exact location of these cleavages; for the crystal had the prism well defined, with terminal planes, and a basal cleavage. The result just given was verified, although the measurement could not be so exactly made as upon the first fragment. The zone readings were: g', 0°; cleavage, 24°; e1, 67°; e', 114°; g', 176°; cleavage, 337° to 340°. It now becomes a matter of interest to discover these cleavages upon other members of the group. A rounded grain of olivine washed from the lava of the Sandwich Islands, was cleaved, and presented a bright surface judged to be the cleavage plane g'. This gave three reflections, the readings for the instrument being: 0°; 6°; 13°, approximatively. In olivine in lava, also from the Sandwich Islands, there were indications of cleavages about the plane g', but they are not well defined, while, parallel to g', this variety cleaves readily; although the surfaces developed are not brilliant.

Besides the interest these cleavages have crystallographically, they may be found to have an important bearing, in comparing some members of this group, which do not occur in well defined crystals.

ART. III.-Notes on some points in the Structure and Habits of the Paleozoic Crinoidea; by F. B. MEEK and A. H. WORTHEN, of the State Geological Survey of Illinois.

THROUGH the kindness of Mr. Charles Wachsmuth, of Burlington, Iowa, we have recently had an opportunity to examine some unique and exceedingly interesting specimens of Carboniferous Crinoids, presenting parts of the structure of these animals, in some instances, never before observed, so far as we are at this time informed. In a few instances, these specimens show internal organs entirely free from the matrix, and although like all the other solid parts of these curious creatures, composed of numerous calcareous pieces, really surpassing in delicacy of structure the finest lace-work, and so frail that a touch, or even a breath, might almost destroy them.† Some of these specimens we propose to notice here, but, before proceeding to do

* From the Proceedings of the Acad. Nat. Sci. Philadelphia, 1869, p. 323. By Mr. Wachsmuth's permission, we have prepared for future publication, drawings of all of these instructive specimens.

so, we avail ourselves of this opportunity to express our thanks to Mr. Wachsmuth for the zeal, industry, skill and intelligence he has brought to bear, in collecting and preparing for study, such an unrivaled series of the beautiful fossil Crinoidea of this wonderfully rich locality. Some idea of the extent of his collection of these precious relics may be formed, when we state that of the single family Actinocrinidæ alone, after making due allowance for probable synonyms, he must have specimens of near 150 species, or perhaps more, and many of them showing the body, arms and column.

It is also due to Mr. Wachsmuth, that we should state here that he is not a mere collector only, but that he understands what he collects, and knows just what to collect, as well as how to collect.

Below we give substantially some notes of observations made in his collection, followed by some remarks on other specimens at Springfield:

1. Synbathocrinus Phillips.-Some of Mr. Wachsmuth's specimens of a species of this genus show that it is provided with a long, slender, pipestem-like ventral tube, or proboscis, apparently equaling the arms in length. Also, that a double row of minute alternating marginal pieces extends up within the ambulacral furrows of the arms, apparently all their length. We are not aware that these characters have been hitherto noticed in any of the publications on this genus. It will be seen, however, farther on, that minute marginal pieces probably occupied the furrows along the inner side of the arms of other types of Crinoidea, as well as this.

2. Goniasteroidocrinus Lyon and Casseday.-Some unusually fine specimens of the typical species of this genus (G. tuberosus) in Mr. Wachsmuth's collection, from Crawfordsville, Ind., show the slender pendent arms much more distinctly than any we had before seen, and from these it seems evident that these arms are stouter than we had supposed, and that there are not more than five or six of them to each of the ten openings. In the specimen figured by us on page 220 of the second volume of the Illinois Reports, these arms were only imperfectly seen by working away, with great difficulty, the hard matrix between two of the produced rays of the vault, which we have termed pseudobrachial appendages, or false arms. In clearing away the matrix of this specimen, we had cut just far enough to expose the edges of the arms on each side of the deep ambulacral furrow, so that each of these edges presents the appearance of being a separate and distinct, very slender arm, composed of a single series of pieces, and without any ambulacral furrow on the outer or ventral side; whereas there is a well-defined