Vol. 50.] STRUCTURE OF CARBONIFEROUS DOLERITES AND TUFFS. 617 



which runs in parallel lines throughout tho crystal. The fibres 

 have a light-yellow colour at first, which deepens into a reddish- 

 brown or blood-red as the decomposition proceeds : they polarize 

 brilliantly and show sometimes a faint pleochroism. Sections 

 parallel to the cleavage-planes give in convergent light a nearly 

 uniaxial interference -figure, and the plane of the optic axes is per- 

 pendicular to the direction of the fibres. According to his illus- 

 tration, the fibres appear to run at right angles to the length of 

 the olivine-crystal. Part of the olivine is often unaltered. In one 

 case he thinks that this mica-like mineral is probably a foliated 

 crystalline form of serpentine, namely, thermophyllite. 



Lawson 1 describes a similar mineral which he does not consider to 

 be an alteration-product of olivine (no fresh olivine is found in his 

 rocks) and to "which he gives the name of ' iddingsite.' It has the 

 same crystalline form as olivine, and the planes of cleavage correspond 

 to the macropinacoid of that mineral. Cleavage-plates are biaxial, 

 the plane of the optic axes is parallel to the C axis of the olivine 

 and perpendicular to the cleavage-planes. The interference-colours 

 are almost like those of muscovite. In thin sections the mineral 

 ranges from a deep chestnut-brown to a citron-yellow or clear 

 yellowish-green. As in the case of the mineral described by Iddings, 

 fragments heated in hydrochloric acid soon lose their colour, the 

 iron being extracted, but their optical properties remain unaltered. 

 As a result of qualitative chemical tests Lawson describes iddingsite 

 as a hydrated non-aluminous silicate of iron, lime, magnesia, and 

 soda. 



Mr. Teall showed me a thin section containing iddingsite. This 

 mineral had outlines similar to those of olivine, but appeared almost 

 opaque in thin sections, and more like haematite. The Derbyshire 

 mineral is always very transparent. 



Mr. Allport 2 describes a dolerite from Duncarnock in which the 

 olivine is " partly converted into haematite,'"' the exterior being of 

 a light red colour, often iridescent, and splitting into thin laminae. 

 I have examined his specimens now in the Natural History Museum, 

 South Kensington. In several thin sections are pseudomorphs 

 consisting of a fibrous or lamellar dichroic mineral something like 

 the Upperwood pseudomorphs. In a few cases they show riDgs, 

 and the arms of a cross in convergent light. (See thin sections, 

 nos. 1523, 1524, 1526, Nat. Hist. Museum.) 



The Potluck pseudomorph is biaxial, with a very small axial 

 angle, has negative double refraction, and often the colour and 

 strong double refraction of biotite in thin sections. As in the case 

 of iddingsite, it behaves generally as a crystallographic individual, 

 and not as an aggregate. The cleavage- planes are also, in every 

 case where I have been able to apply any test, in thin sections, 

 parallel to the macropinacoid of the original olivine. 



1 ' The Geology of Carmelo Bay,' University of California, Bulletin of the 

 Department of Geology, May 1893, Lawson and Posada. 



- ' On the Microscopic Structure and Composition of British Carboniferous 

 Dolerites,' Quart. Journ. Geol. Soc. vol. xxx. (1874) p. 541. 



