OF ARTS AND SCIENCES. 



291 



The amygdules have generally, not always, sharp outlines. Some 

 are of unaltered prehnite. Others, which have evidently consisted of 

 prehnite with long radiating structure, are more or less altered to a soft, 

 homogeneous, impelhicid, green substance, which seems to be structure- 

 less, or to polarize the light only very feebly. It is a frequent altera- 

 tion product of prehnite in these rocks. Some of the amygdules are 

 wholly changed to chlorite. In most of the amygdules containing 

 altered prehnite, tJie orthoclase occurs, showing aggregate polariza- 

 tion, and intimately associated with small fragments of prehnite, par- 

 tially altered to the green substance ; and while these are scattered 

 through the orthoclase aggregate, they show in polarized light that they 

 are merely remnants of a formerly continuous radiating mass of preh- 

 nite, the rest of which has been changed to orthoclase. As a rule this 

 change has been accompanied by a large diminution of volume, result- 

 ing in a central empty cavity, into which the feldspar crystals project 

 freely crystallized, and show there integral polarization. 



The appearances seem to indicate that the pseudomorphs of ortho- 

 clase were formed after the partial destruction of the prehnite. 



The pyroxene of the matrix has, in places, been altered to a bright 

 yellowish-green, soft, double-refracting substance : none of the charac- 

 teristic pseudomorphs were seen. 



The paragenesis, in so far as it is determinable, is — 



MATRIX. 



(I.) 1. Plagioclase. 



(11) 



2 Pyroxene. 



(III.) Chlokite. 



(IV.) 



Soft, green, double-refract- 

 ing substance. 



AMYGDULES. 



Prehnite. 



I I Green EARXH-like 



Chlorite. product. 



Orthoclase. 



The melaphyre proper, which forms the lower tnember of bed No. 64, 

 is a dark green, almost black, cryptocrystalline rock, which is easily 

 scratched with the knife. Under the microscope, it is found to consist 

 chiefly of plagioclase in very small crystals, a soft, green mineral, prob- 

 ably pseudomorphous after olivine, minute grains of augite, and occa- 

 sional small, often wedge-shaped, occurrences of a green soft substance, 

 occupying the interstices between feldspar crystals. 



The feldspar appears, from optical measurements in the zone 0: it, 

 to be anorthite. 



