264 PROCEEDINGS OF THE AMERICAN ACADEMY 



usually finer grained and almost aphanitic in texture. Where it is 

 least altered, the fresh fracture has less lustre, but the characteristic 

 lustre-raottlings are present to the same extent, thongh in smaller 

 spots. 



An instance of a highly altered form of this variety occurs at the 

 shaft on Mabb's vein near Houghton. The specimens are from the 

 immediate neighborhood of the fissure vein ; and the one on which 

 tlie f.illowing observations vs^ere made represents the contact: — 



It is about 5x4 inches, and carries at one end | of an inch thick- 

 ness of the vein, in the form of a parallel arrangement of calcite-quartz 

 seams, separated by thin chlorite layers. 



lu close connection with this altered form there is an intermediate 

 stage which has an uneven fracture, and is distinguishable from the 

 "greenstone" only by a somewhat finer texture, and the smaller size 

 of the lustre-mottlings. The magnet extracts a little magnetite from 

 the powder. In thin sections, the characteristic structure to which the 

 lustre-mottlings are due appears at once between crossed nicols, and- 

 the same crowding of olivine and magnetite individuals into the space 

 surrounding the large pyroxene areas. Botli the feldspar and pyroxene 

 appear to have suffered but little from alteration ; but the chrysolite 

 individuals are all altered, and the unindividualized substance is 

 changed to a green non-dichroitic substance. 



The more altered form has an even to semi-conchoidal fracture. 

 The freshly broken surface is formed of nearly round, green spots, 

 about ^ inch in diameter, in a connected network of brown. 



Fragments of the rock effervesce strongly in muriatic acid. Its 

 powder does not yield magnetite. 



The structure that in the less affected rock causes simply lustre- 

 mottling is, in this altered form, marked by a decided color-mottling. 



In thin sections, the plagioclase crystals show still, to a great extent, 

 the twin striation in polarized light. 



The olivine individuals are changed to a slightly yellowish green 

 substance with brown stained cracks, and with the outer portion also 

 stained brown. They are very soft under the needle. 



Examining these with a strong glass, on a fresh fracture of the rock, 

 they show a well-marked cleavage in one direction, — an irregular 

 fracture in all others, — and have a dark red color with somewhat 

 wax-like lustre. They blacken before the blowpipe, and fuse with 

 difficulty on the edges of very small fragments. 



In the thin sections, many of the individuals are so stained, in part 

 or throughout, by oxide of iron as to be wholly opaque ; but by etching 



