354 



SCIENCE. 



[Vol. XX. Nj 516 



ON INTERGROWTHS OF HORNBLENDE WITH AUGITE 

 IN CRYSTALLINE ROCKS. , 



BT WM. H. HOBES, MADISON, WIS. 



The question of the primary or secondary origin of hornblende 

 in a number of types of eruptive and metamorphio rocks is one 

 of the most difficult to answer of any that are raised by their 

 study. The number of varieties under which the calcium mag- 

 nesium iron silicates that we call hornblende occurs, makes it a 

 somewhat JiflScult matter to correlate results. The term " Ural- 

 ite," which Gustave Rose applied to a tibrous hornblende from the 

 Urals, which waspseudomorphictohornblende. has sometimes been 

 loosely applied to any variety of hornblende which may be supposed 

 to have this origin. Other observers have distinguished ''com- 

 pact hornblende" from uralite. and have also carefully stated tlie 

 character of the mineral's absorption. Urslite vvlien applied with 

 the proper restrictions, is always an alteration product of pyroxene. 

 It is a matter of the commonest occurrence to find basic eruptive 

 rocks, particularly diabase, in which the alteration of augite to 

 this mineral can be clearly seen. As regards the compact variety, 

 it has been described as secondary to augite by Hawes,' Irving 

 and Van Hise,'' and Williams.^ 



In the beautiful monograph on the " Eruptive Rocks of Electric 

 Peak and Sepulchre Mountain, Yellowstone National Park,"* Pro- 

 fessor J. B Iddiugs devotes considerable space to the description 

 of very interesting intergrowths of augite and hornblende, both 

 in diorites and glassy rocks The author uses the opportunity to 

 raise a strong doubt as to the secondary nature of compact horn- 

 blende in those cases in which it has been described. Because of 



the deservedly wide reputation of Professor Iddings, bis generali- 

 zations regarding this point will be received with much considera- 

 tion. It has seemed to the present writer that Mr. Iddings should 

 have made mention of earlier descriptions of intergrowths of these 

 minerals where the primary nature of the hornblende has been as 

 clearly demonstrated as in the cases he describes. 



Parallel intergrowths of augite and hornblende have been fre- 

 quently observed in eruptive rocks. Teall,' Rohi-bach," myself,' 

 and probably others have figured them. Rohrbach described in- 

 tergrowths in a teschenite, from the Teufelsgrund, in which the 

 hornblende has its own outlines and is sharply outlined from 

 augite. Chemical analysis showed an essential difference in com- 

 position between the augite and hornblende. 1 have described 

 very similar growths in the augite diorite fi-om Medford in Massa- 

 chusetts. Here the hornblende is the brown variety and the 

 augite the pink variety common in diabases. That the hornblende 

 is primary is shown not only by its idiomorphic character, but 

 also by tiie fact that the augite is sometimes almost entirely altered 

 to clorite, the hornblende remaining fresh. Professor Iddings's 



I Mineralogy aad Littiology of New Hampshire, Plate vil.. Fig. 1. 



3 Geology of Wisconsin, Hi, 170 ; iv., 662. American Journal of Science (3), 

 sxvl., 29. 



= Ibidem, xxviii., 359-368. 



^ Extract from the Twelftli Annual Report of the Director of the U. S. Geo- 

 logical Survey. 



^ Quarterly Journal of the Geological Society, London, si.. 653, Plate xxis., 

 Fig. 3 



* UeDor die EruptlV;^e8t6ine in Gebiete der fchlesischen-mahrischen 

 Kreldefurmatlon. Min. u. petrog. Mitth , vil., 24, Plate i.. Pigs. 1-7. 



' On the Petrographical Characters of a Dike of Diabase in the Boston Basin. 

 Bull. Comp. Zool., Harv. Coll., xvi., 10, Plate 1., Pig. 2. 



conclusions will doubtless go far toward correcting any tendency 

 to describe compact hornblende as secondary when the principal 

 basis for it is the analogy with uralite, as his observation of an 

 instance of hornblende altered to augite brings into the study of 

 the relations of these minerals a new complication. 



I have recently observed some rather unusual intergrowths of 

 augite and hornblende in a rock from the "Cleveland Gold Mine" 

 in New Marlboro, Mass. The rock is largely composed of these 

 minerals, but is slightly calcareous and is apparently a phase of 

 crystalline limestone. Nearly all the crystals represent inter- 

 growths, the hornblende generally predominating and enclosing 

 the augite, which is of irregular outline and oriented like the horn- 

 blende. Prismatic sections show a wide divergence in the ex- 

 tinction angles, and the hornblende is light-green and pleochroic, 

 while the augite is almost colorless. The intergrowth figured is 

 interesting because the augite in this instance completely sur- 

 rounds the hornblende, a structure that I think is rare, as I have 

 not seen it described. The section is nearly perpendicular to the 

 c axis, since the cleavage angle in the augite was measured as 

 89°-90°, and that of the hornblende as 125°. While sharply con- 

 trasted by differences in their color and cleavage angle, the two 

 minerals are more markedly distinct in polarized light. I have 

 noticed other instances of intergrowths of these minerals within 

 the same area, but this is the only one where hornblende was seen 

 to be entirely enclosed by augite. 



An examination of the section figured will show how intricate 

 is the intergrowth. Islands of augite are enclosed within the 

 hornblende. A somewhat pronounced parting parallel to the 

 clioo-pinacoid passes through both minerals. 'J here seems to be 

 considerable similarity between intergrowths of these minerals 

 and the quartz which is so often enclosed within the feldspar of 

 pegmatites. The hornblende, like the feldspar, is most frequently 

 the enclosing mineral, and in the instance described by Rohrbach 

 it is, like the feldspar, the' more basic of the two minerals. 



OPTICAL ANGLE AND ANGULAR APERTURE. 



BY ALFKED C. LANE, MICHIGAN MINING SCHOOL, HOUGHTON, MICH. 



The observation of the brilliantly-colored images which are 

 given by various crystals, natural and artificial, in polarized light 

 is of considerable diagnostic value. The apparent breadth be- 

 tween the two branches of the hyperbola which may be seen in 

 the image given by many biaxial substances, e.g , white mica, is 

 dependent upon the optical angle, — a constant characteristic of 

 them. The relation between this breadth and the "optical angle 

 in air" (2E) is usually found by noting the apparent breadth in 

 the case of a plate whose optical angle is somehow otherwise 

 known (see Iddings's translation of Rosenbusch's "Microscopic 

 Physiography," also Czapski ib the Neues Jahrbuch fiir Mineralo- 

 gie, etc., 1892, supplementary vol. vii.). 



I wish to describe briefly the very simple method that I use for 

 determining said relation, which also may be used to determine 

 the angular aperture of the objective. 



It works well in class, and the only reason why it has not long 

 ago been adopted seems to be that the German microscopes on 

 which the technique of the subject has been developed are not 

 built to admit of it. But any microscope whose mirror bar is 

 graduated to measure the obliquity of the light will do. 



We will suppose, then, that we have such a microscope, that 

 above and below our plate of mica we have nicols, below it a 

 strong condenser, and above a short-focus objective. 



We may use a camera and project the image with its hyperbola 

 on paper, but we will suppose that instead of that we use a Ber- 

 trand lens, which slips into the tube between eye- piece and ob- 

 jective, and with the former makes a compound microscope which 

 magnifies the image given by the objective alone. We will also 

 use a micrometer eye-piece. To measure the distance between 

 the hyperbola branches, the micrometer scale must run diagonally. 

 After noting the position of the branches on the scale, turn it till 

 it runs right and left, the same way that the mirror swings. 

 Then, without altering the distances between Bertrand lens, ob- 

 jective, and eye piece, lower the whole tube until the front of the 



