OP ARTS AND SCIENCES. 271 



the finished state no trace of the feldspar is visible except the outlines. 

 The pseudomorph then shows an aggregate polarization due to a con- 

 fusedly felted mass of minute chlorite tufts. The substance is poorly 

 chai'acterized. 



The green substances which I have described so unsatisfactorily, as 

 being undoubtedly pseudomorphous after residuary-base, pyroxene, and 

 plagioclase, have in themselves no physical properties, recognizable under 

 tlie microscope, which are sufficiently persistent to invariably charac- 

 terize them respectively. Often the best means of distinguishing be- 

 tween them is in the internal structure of the aggregate, since this is 

 intimately connected with certain physical characteristics of tlie original 

 mineral which predetermined the mode of growth and gradual arrange- 

 ment of the secondary product. In other instances, the presence or 

 absence of cracks stained with iron oxide are characteristic. 



Thus the change of the residuary base has resulted in a tendency to 

 form bands parallel to the contour of the area, in a manner that indi- 

 cates a gradual progress along concentric shells from the circumference 

 inward or the reverse ; while in the feldspar the growth appears to 

 have begun without any regularity throughout the cleavage and 

 twinning planes of the crystals. The pseudomorphs, after pyroxene, 

 are almost invariably defined by the red-stained cracks, and slight mix- 

 ture of brown in the green. 



There is another occurrence of chlorite in which the progress of 

 growth is from within outward. Throughout the pseudo-amygdaloid 

 occur grains of chloritic substance, which in places reach a diameter of \ 

 to § inch, with often more or less irregular outlines, often nearly round 

 or oval. These consist of a dark green mineral with II = 2.5, which 

 fuses. B. B. at 3 — 3.5 to a black magnetic slag.* In different beds, its 

 texture under a hand-glass varies fro.m amorphous to finely scaly. In 

 thin sections, in polarized light, the substance often resembles closely that 

 in the pseudomoi-phs after plagioclase, except in that it shows evident 

 growth from within outward. There is no defined wall, as of a pre- 

 existing cavity, but the chlorite often sends out long arms, which sur- 

 round or penetrate the adjoining primary constituents. In this manner 

 the chlorite-like pseudomorphs after plagioclase and pyroxene, etc., are 

 sometimes incorporated into tliese pseudo-amygdules. Very often one 

 of these bodies has a large central area filled with closely packed radi- 

 ating spheres, surrounded by fragments of a once continu(jus band with 

 cross-fibrous structure, wdiich evidently once formed the outer limit; 



* For chemical analyses of this chlorite, see beyond under " Eed No. 87." 



