OP ARTS AND SCIENCES. 267 



centric layers, and while the central ones are complete there was 

 evidently not room for tlie outer layers on one side, owing to the prox- 

 imity of the face of a calcite crystal against which they end abruptly. 



In some instances, the quartz shows a comby structure, and the inter- 

 stices between the opposing ends — the median line — are occupied by 

 a little green, chloritic substance. 



The process of growth of the small veins near the fissure vein is 

 well defined under the microscope. Throughout the body of the speci- 

 men, the calcite is confined to the pseudomorpiis after pyroxene, and to 

 the minute crooked veinlets. But near the end bearing the vein frag- 

 ment, a greater regularity in the form of the minute veins is apparent, 

 and is evidently due to the lesser resistance offered in the direction of 

 the fissure. The veinlets unite, to form wider veins ; and in doing this 

 the tributary always enters between the larger veinlet and the rock, 

 and maintains this position. Each member of the compound vein is 

 marked by its independent crystallization, and by the narrow borders 

 of impellucid substance, and the whole appearance is best comparable 

 to the map of a glacier which has received several tributaries, each 

 marked by its lateral moraines. 



The greater facility for di-ainage indicated in this structure has had 

 for effect, near the vein, the removal of the calcite from the pseudo- 

 morpiis after pyroxene, which appear nov/^ to be filled with chlorite 

 and quartz. 



Minute particles of native copper occur in these veinlets. An exam- 

 ination of fragments containing the copper flakes, under the microscope 

 in reflected liglit, showed the metal to be younger than the quartz, for 

 it adapts itself to the contours and cracks of the latter. 



Throughout all this change the plagioclase appears to have suffered 

 little, except in the immediate contact with the fissure vein, where it 

 has lost in most individuals the evidence of twinning in polarized light. 

 It is also, perhaps, affected in the immediate contact of its crystals with 

 the veinlets, but it is generally so fresh that it cannot have contributed 

 appreciably to the changes that the rock has undergone. 



The first change — that affecting the chrysolite and the residuary base 

 — may have been simply of the nature of a hydration of these, accom- 

 panied, in the former, by a separation and oxidation of the iron. This 

 represents the condition of the "greenstone," in which this change is 

 partially accomplished, and of the similar " mottled traps " throughout 

 the copper district, in which the alteration of the chrysolite and residuary 

 base is, probably, generally complete. 



The next change has affected the magnetite, and still more the pyro- 



