270 PUOCEEDINGS OF THE AMERICAN ACADEMY 



The next step has been the change of the chrysolite. In the so-canod 

 greenstone, this has been only partial; but generally in the chi'y>- 

 olite-bearing beds it is complete. The result in thin sections is a 

 fiiintly green substance, soft under the needle, and surrounded, within 

 the orifinal contours of the crystal, by a more or less opaque deposit 

 of iron oxide, which also traversed it in fissures. The green substance 

 shows by a well-defined cleavage in one direction that it is in thin 

 laminas. Between crossed nicols, these lamime have an appearance of 

 twin structure, polarizing the light in alternate lines of brilliant red 

 and green. The whole pseudomorph becomes dark when the cleavage 

 is parallel to a nicol plane ; and some individuals, probably cut parallel 

 to the cleavage, remain dark through a revolution of the stage. The 

 substance is, therefore, very probably uniaxial. It has very appreciable 

 absorption for intensity, and very feeble for color. 



The augite was the next to undergo change. Generally in any thin 

 section of the lower portion of a bed, a considerable proportion of 

 the pyroxene is fresh, either throughout whole individuals or in parts 

 of these. 



In thin sections, by ordinary transmitted light, the psendomorphous 

 product is translucent, faintly light green, with a tinge of brown. Be- 

 tween crossed nicols, in its most characteristic form, it shows irregular 

 lamellar a<r"i"egate polarization. It is very soft under the needle, and 

 is traversed by red stained cracks corresponding to the irregular fissures 

 in the parent pyroxene, and it is by these, together with the structure 

 as seen in polarized light, that it is generally best distinguished from 

 the product after residuary magma-base. 



The mineral forming these pseudomorphs is very probably the result 

 of a process which has removed lime and some iron, magnesia, and 

 silica from the pyroxene and brought in water, and it is, probably, poor 

 in alumina. 



The pla^ioclase is generally the last constituent that has been altered 

 to any great extent. The most usual product is chloritic. It is very 

 usual to observe very minute particles of a green, a[)parently structure- 

 less substance, suspended in the interior of the feldspar in such a man- 

 ner as to render the supposition quite possible that they are due to an 

 alteration of enclosed particles of hyaline base. But an actual pseu- 

 domorphism of a chlorite after plagioclase is observable on a large scale. 

 In the first stages, small tuft-shaped particles, consisting of laminae or 

 fibres radiating from a point, occur scattered through the interior of 

 the feldspar, and these may wholly occupy a considerable portion of a 

 crystal, while the rest still shows twin striation in polarized light. lu 



