612 MR. H. H. ARN0LD-BEMR0SE CKN THE MICROSCOPICAL [Nov. 1 894, 



so that when a hand-specimen of rock is much decomposed it is 

 easy to find the olivine-psendomorphs with a lens. Fresh olivine 

 occurs in twenty-eight thin sections from eleven outcrops. In most 

 of these the greater part is entirely unaltered, except for a slight 

 discoloration in the cracks, most of which are rectilinear. The 

 curved cracks which are present, in some cases combined with an 

 alteration on the outside of the mineral, result in large granules of 

 olivine without crystalline form. Some of the crystals are corroded 

 and eaten into by the groundmass, and thus contain felspar-laths 

 and augite in granules. 



In some specimens the size of the crystals varies considerably. 

 In one from a basalt near Chelmorton, outcrop 19, there are a 

 number of small ones perfectly bounded, and embedded amongst the 

 augite and felspar of the groundmass. One measures -08 x "06 mm., 

 and shows a bisectrix perpendicular to the section. The largest 

 measure about "55 x "4 mm. 



The olivine often occurs in groups up to 5 mm. in diameter, con- 

 sisting of two or three or even eight or nine individuals. A speci- 

 men from Tideswell Dale Quarry, outcrop 17, contains a group of 

 eight individuals which fit closely together but extinguish in 

 different positions. Two of them appear to form a twin, the angle 

 between their directions of extinction being 70°. Such groups are 

 common. 



A specimen from a basalt near Blackwell, outcrop 14, shows an 

 interpenetrating twin, the two individuals being clearly marked in 

 polarized light. The acute angle of one is about 65°, and that of 

 the other about 73°. Both sections show a bisectrix, though not 

 very clearly. The greater axis of elasticity of the section is in 

 both individuals at right angles to the long axis. The angle between 

 the long axes of each is 55°. 



In another case the angle between the directions of extinction of 

 the two individuals is 57°. Several cases occur like the following. 

 Two individuals almost rectangular in section, the dome faces 

 absent, are divided by the trace of the plane of composition. The 

 section of one individual is at right angles to an optic axis, and that 

 of the other extinguishes at an angle of 15° with the dividing-line. 

 In another case one extinguishes parallel to its length, and the 

 other at 25° with the dividing-line. 



The olivine is replaced by iron oxide, calcite, serpentine, chlorite, 

 and a mica-like mineral. The iron oxide is generally opaque. In 

 one slide it is transparent dull red, and appears the same in 

 polarized as in ordinary light. It is not dichroic, and shows no 

 axial figure in convergent light : it is thus distinguished from 

 olivine coloured by iron. The calcite is sometimes clear and com- 

 posed of large crystalline pieces, at others cloudy, and shows 

 aggregate polarization. The only pseudomorphs after olivine which 

 require any description are two which are found to a very large 

 extent in several outcrops. For convenience, I have called them the 

 Potluck and Peak Forest pseudomorphs. 



