BASALT. 389 



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

 dark red altered olivine yielding a negative interference cross with OIK- dark ring. 



Prof. Eosenbuscli describes a similar occurrence in the melaphyre from Asweilen, 

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

 appearance of specular iron. They have partly the form of oliviiie and show by well 

 preserved remnants of this mineral that they are pseudomorphs after the sa inc. In 

 other cases, however, such 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 pleochroism, and a position of 

 the axes of elasticity parallel and at right angles to the cleavage. One can scarcely 

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

 pleochroism in specular iron." ' In the melaphyre from Reidelbacher Hof nea r Wadrill 

 and the olivine-diabase from Eckelhausen and trotinesweilen on the left bank of the 

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

 It is very common in the basalts of the Fortieth Parallel collection, as noticed by Prof. 

 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 different proce-M- 

 are never found 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 parallel orientation of all its parts, for the extinction of light 

 is the same throughout and the interference figure 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 Valley, Truckee Range, 2 were 

 subjected to hot concentrated hydrochloric acid, and afterwards placed under the 

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

 to red oxide of iron, and to remain light yellow. The tabular fragments gave for interfer- 

 ence figures hyperbolas, 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 00, leaving no reasonable doubt that the substance in 

 this case is a nearly colorless, mica-like mineral, colored by red oxide of iron, 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 

 probable from the fact that most of these basalts are so fresh, with the decomposi- 

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

 change is not likely to have taken place, and that a simple hydration and oxidation of 

 a very ferruginous olivine would supply all the chemical elements necessary to trans- 

 form it into anhydrous unisilicate of magnesia and ferric oxide; besides which is flu- 

 fact that the optical properties of the mineral in question correspond to those given 



'H. liosenlmsi'h. Mikroskopi*. -In- rii\i<i|;ro]iliii<. Stuttgart, 1877, p. 400. 

 'Forth-Ill paniUcl nilli-.-timi, No. 2X129. 



