294 PROCEEDINGS OF THE AMERICAN ACADEMY 



On the Mesnard property, fiftij feet west of the Houghton Conglom- 

 erate, there is a pseudo-amygdaloid melaphyre, which I mention here, 

 because the changes that have taken phice in it belong more pro)ierly 

 in the stage of development found most commonly among the amygda- 

 loids. 



In a fine-grained, greenish-brown matrix lie pseudo-amygdules, which 

 sometimes attain a diameter of one inch or more. On breaking these, 

 we find a soft, dark-green compact substance, speckled with white and 

 red, the latter from orthbclase, which, in places, shows the combination 

 of a prism with the basal plane. 



In thin sections, the matrix is found to be of the same type as the 

 "greenstone," though coarser grained, and less unaltered. Here, as 

 there, the chrysolite grains are crowded into the spaces intervening 

 between relatively large areas, each of which consists of a pyroxene 

 individual, enclosing many plagioclase crystals. The constituents are 

 plagio'clase crystals, generally very fresh, much unaltered pyroxene, 

 and relatively few of the characteristic pseudomorphs after this, but 

 many after chrysolite, associated with considerable specular iron, also 

 from the chrysolite in part. Besides these, there are numerous pseudo- 

 amygdules of chlorite and some pseudomorphs of chlorite after plagio- 

 clase. Optical measurements in the zone 0: n, appear to indicate 

 auorthite.* 



The large pseudo-amygdules were undoubtedly once prehnite ; but of 

 this there now remain three products. Rounded areas of a soft, opales- 

 cent, white, translucent substance show a minutely scaly aggregate polar- 

 ization, and revolve equally bright, without apparent change, between 

 crossed and parallel nicols : this is probably a clay. These areas are 

 fringed with a hard, white, or pink-white mineral, showing granular 

 aorcrreirate polarization, and belon<iing to the orthoclase. Often this 

 feldspar occupies a large part of the white areas, and is then distin- 

 guished from the soft substance by its coarse aggregate polarization 

 of light. These areas of soft, white substance and orthoclase are sur- 

 rounded by a green chloritic mineral, which is made up of minute 

 spheres with radiating structure, and which resembles that of the 

 smaller pseudo-amygdules in the matrix. 



The appearance of the sections is such, that the soft, white alteration 

 product of the prehnite seems to be the starting-point for the formation 

 of both the orthoclase and the chlorite. 



Paragenesis : — 



