600 



SCIENCE, 



[Vol. I., No. 21. 



OPTICAL RESEARCHES ON GARNET. 



It has been for a long time known that all garnets, 

 as well as some other isometric minerals (boracite, 

 analcite, alum, senarraontite, etc.), do not show the. 

 action on polarized light which would be required by 

 substances crystallizing in the isometric system ; and 

 to find out the causes of these optical variations, and 

 the laws which govern them, C. Klein has examined 

 (Jalirb. mill., 1883, 87) as many as three hundred and 

 sixty different garnet sections, cut parallel to differ- 

 ent crystallographic planes, and from variovis locali- 

 ties. His researches do not indicate that because 

 garnets frequently show these optical variations we 

 should refer them to some system of crystallogra- 

 phy other than the isometric; for garnets from the 

 same locality often show a great variation in optical 

 properties, some crystals being isotrope throughout, 

 others in part uniaxial or biaxial. Others, on the 

 other hand, have tried to explain the optical varia- 

 tions by i-egarding the various isometric forms as 

 made up of numerous prisms, either uniaxial or 

 biaxial, united at the centre, and whose bases make 

 up the external crystal faces. Others regard the gar- 

 net substance as triclinic, and the various optical 

 properties as the result of repeated microscopic twin- 

 ning of the same. 



The chemical composition does not influence the 

 optical structure of the crystals, because the same 

 optical phenomena are observed in garnets of differ- 

 ent composition; and in garnets of the same compo- 

 sition, but with different form, varying optical struc- 

 tures are observed, even among crystals from the 

 same locality. The form, however, in which the 

 various garnets occur, governs the optical structure. 

 Thus, in the octahedral garnets from Elba, what is 

 called the octahedral structure is noticed. A section 

 from this garnet cut parallel to an octahedral face, 

 examined in parallel polarized light with crossed 

 nicols, shows a triangular centre, which remains 

 dark, and three fields on either side, which are alter- 

 nately dark and light as the section is turned, being 

 dark when one of the sides of the triangle becomes 

 parallel to the plane of either of the nicols. In con- 

 vergent polarized light, the centre shows the dark 

 cross of a uniaxial crystal, while from each of the 

 three sides a dark bar runs out into the side-fields 

 at right angles to the edge. This indicates a crys- 

 talline structure made up of eight uniaxial prisms 

 united at the centre of the crystal, and whose bases 

 form the eight faces of the octahedron. A section 

 cut near the centre of the crystal shows six of these 

 prisms radiating out, while the upper and lower ones 

 have been, of course, cut away. What is called the 

 dodecahedral structure is observed on pure dodeca- 

 hedrons. A section cut parallel to a dodecahedral 

 face shows, in convergent polarized light, the appear- 

 ance of two optic axes whose plane lies parallel to 

 the longer diagonal of the rhomb. The tetragonal- 

 trisoctaiiedral "structure observed on crystals of that 

 form shows, in sections parallel to the trisoctahedron 

 faces in convergent polarized light, the appearance of 

 two optic axes with very slight divergence, indicating 

 a crystalline structure made up of twenty-four nearly 

 uniaxial prisms united at the centre, and whose bases 

 are the faces of the trisoctahedron. The plane of 

 the optic axes is normal to the symmetry diagonal 

 of the trisoctahedron face. In tjie hexoctahedron 

 structure the sections show a biaxial structure, and 

 the plane of the optic axes is very variable. By 

 making and examining artificial gelatine crystals, the 

 author was able to imitate many of the optical varia- 

 tions ; and these seemed to be related to a contraction 



working along the edges of the crystal, and normal to 

 its faces. The greater the contraction along the 

 edges in relation to that normal to the faces, so 

 much greater will be the double refi'acting power of 

 the crystal. The cause, then, of the optical varia- 

 tions observed in many garnets seems to be tension, 

 caused by unequal contraction, and this being in- 

 fluenced largely by the external elements (edges) of 

 the crystal gives to each form its peculiar optical 

 structure. S. L. Penfield. 



GEOLOGICAL NOMENCLATURE. 



The following resolutions concerning nomencla- 

 ture, coloiing, etc., were voted by the recent interna- 

 tional geological congress : — 



I. Nomenclature. 



The elements of the earth's crust are the mineral 

 masses (masses minerales). 



The mineral masses, regarded from the point of 

 view of their nature, take the name of rocks.^.iCon- 

 sidered from the point of view of their origin or 

 mode of formation, they are to be called formations. 



a. Stratigraphical divisions. 

 Regarded from the point of view of their age, 

 mineral masses may be subdivided according to the 

 following rules: — 



1. The word group (groupe) is applied to the three 

 or four great divisions. Ex. : Secondary group. 



2. The divisions of the groups are designated by 

 the word system. Ex. : Jurassic system. 



3. The divisions of systems of the first grade are 

 designated by the word series (serie), or by the terras 

 section or ahtlieilung. Ex. : Lower oolitic section or 



4. The divisions of systems of the second grade 

 are designated by the word etage, or by the corre- 

 sponding terms, piano (Italian), viso (Spanish), 

 stage (English), stvfe (German), etc. Ex.: £tage 

 bajocien. 



5. The divisions of systems of the third grade are 

 designated by the term assise, or by its strict equiv- 

 alents in the different languages. Ex. : Assise fit A. 

 Humphresianus. 



6. The French expression couches (beds) may be 

 employed as synonymous with assise. 



7. A certain number of assises combined will bear 

 the name of substage (sous-etage). 



8. The first element of stratified masses is the 

 strate or couche, schicht (German), stratum (Latin 

 and English), strato (Italian), retek (Hungarian). 



b. Chronological divisions. 



9. The word era (fere) is applied to the three or 

 four great divisions of time, corresponding to the 

 groups. 



10. The length of time coi responding to a system 

 will be rendered by the word period (p^riode). 



11. The length of time corresponding to a series 

 (section, s^rie, abtheiluug) will be expressed by the 

 word epoch. 



12. The length of time corresponding to a stage 

 (6tage) will be expressed by the word age. 



II. Colors and signs. 



1. Crystalline schists, I'ose-carmi/ie (by preference); 

 bright rose for the rocks of pre-Cambrian age; paZe 

 rose for those of indeterminate age. 



2. Primary group. Decision referred to the com- 

 mittee of the map of Europe. 



