HASALT. 389 



red alteration found to belong to the second kind of decomposition, one section of 

 dark red altered olivine yieldinu' a negative interference cross with one dark ring. 



Prof. Eosenbnsch describes a siTnilar oeenrreiiee in the melaphyre from Asweilen, 

 among the crystalline ingredients of which, he says, "lie large grains having the 

 appearance of si)ec;ular iron. Tliey have partly the form of olivine and show by well 

 l)reRerved remnants of tliis niincial that they are psendomorphs after the same. In 

 other cases, however, siu'h an origin is not demonstrable; the blood red substance is 

 then either very compact and faintly or not at all translucent (basal section) or else 

 it shows a perceptible, monotonous cleavage, strong ])leochroism, and a position of 

 the axes of elasticity i)arallel and at right angles to the cleavage. One can scarcely 

 consider this body as anythijig else than a blood red mica, for 1 know of no such 

 pleochroism in specular iron." ' In the melaphyre from Keidelbacher Hof near "Wadrill 

 and the olivine-diabase from Eckelhausen and (lounesweilen on the left bank of the 

 Khine, the decomposition of the olivine has resulted in the same red micaceous mineral. 

 It is very common in the basalts of the Fortietli I'aiallel collection, as noticed by I'rof. 

 Zirkel ; but after a careful search through all the thin sections of basalt from that 

 region, with one rather doubtful exception, it appears that the two dillerent processes 

 are neverfouud to have taken place together in the same thin section. The resultant 

 mineral from its optical properties is evidently not a confused aggregate, but a crystal- 

 lographic individual, with j)arallel orientation of all its parts, for the extinction of light 

 is the same throughout and the interference tiguie that of a doubly refracting crystal. 



In order to arrive as nearly as possible at its actual nature, fragments of a similarly 

 altered, porphyritic olivine in the basalt from Truckee V'alley, Truckee Range,'' were 

 subjected to hot concentrated hydrochloric acid, and afterwards i)laced under the 

 microscope, when they were seen to have lost their intense red color, which was due 

 to red oxide ofiron, and to remain lightyellow. The tabular fragments gave for interfer- 

 ence figures hji)ery)olas, which parted only a short distance, indicating a small angle 

 between the optic axes and showing a negative bisectrix. One plate was marked 

 by lines intersecting at (iO"^, leaving no reasonable doubt that the substance in 

 this case is a nearly colorless, micalike mineral, colored by red oxide ofiron, which 

 latter is occasionally seen in well crystallized hexagonal films in the cracks of less 

 altered olivine. That this mineral is a foliated, crystallized form of serpentine seems 

 l)robable from the fact that most of these basalts are so fresh, with the decomposi- 

 tion of the olivine frequentls confined to the weathered surface, that a very radical 

 change is not likely to have t;i]cen place, and that a simide hydration and oxidation of 

 a very ferruginous olivine wcaild sui)ply all the chemical elements necessary to trans- 

 form it into anhydrous nnisilicate of magnesia and ferric oxide; besides which is the 

 ftict that the optical properties of the mineral in ((uestion correspond to those given 



in. Roaeubusch. MikrosUo))isclRi I'liysiograpliii-. Stuttgart, 1877. p. 400. 

 •■'Fortieth paralkl < ..ll,.,ti..n, Nu. 22J29. 



