OF ARTS AND SCIENCES. 269 



of the chrysolite have been changed, while a considerable portion of 

 the chrysolite, and all of the plagioclase, pyroxene, and magnetite, 

 remain apparently quite intact. Tiie farther change whicli this variety 

 has undergone under favorable circumstances, namely, the close prox- 

 imity of a vein, has been already described. I propose now to con- 

 sider the evidence, and, so far as I have been able to trace it, the 

 progress, of the alteration wliich has taken place in the body of the 

 rock, especially in the diabase proper, away from the influence of veins, 

 and where the influx into, and drainage from, a given point, could have 

 taken place only through capiUary cracks and the cleavage planes of the 

 crystals. As I have shown elsewhere, all the beds have very sharply 

 defined hanging walls and foot-walls, while between these limits the 

 rock has, generally, three subdivisions ; namely, a lower — least altered 

 zone — below, a pseudoaraygdaloid in the middle, and amygdaloid at 

 the top. The upper one, and sometimes the middle one, are, in places, 

 wanting. The relative thickness of these subdivisions is very variable, 

 not only in different beds, but also in different parts of the same bed ; and 

 there is no plane of separation between them. This classification is 

 based on the presence and character of the amygdules in the middle 

 and upper division, an4 their infrequent occurrence in the lower. 



The amygdules in the middle portion, and the more isolated ones in 

 the lower, as a general thing, occupy the place of former constituents ; 

 while those of the amygdaloids proper have, to a great extent, been 

 formed in pre-existing cavities of more or less regular form. 



The changes that have taken place in the middle and lower portions 

 of any bed are such as tend to produce a pseudo-amygdaloid. The 

 first and ever-present stage of alteration is caused by the change of the 

 residuary magma-base which fills the interstices between the crystalline 

 constituents, and in places penetrates into, or is enclosed in, the in- 

 terior of these. The physical and cliemical character of this seems 

 to have predisposed it to an easy change. It is now, as a rule, when 

 seen in thin sections, a darker or lighter olive-green substance, and 

 very soft under the needle (hardness not over 2.5). In polarized 

 light it exhibits a fibrous aggregate polarization, and shows well its 

 structure, which is short fibrous, converging towards the centre. The 

 central portion shows sometimes little or no double refraction, but more 

 generally it is filled with very minute polarizing spheres formed of 

 radiating fibres. With one nicol, this substance shows only absorption 

 for intensity. The contours are generally sharply defined by the 

 feldspar and pyroxene crystals, and the result is usually a more or less 

 wedge-shaped form. 



