Kai rbanks.J 



Ana/cite Diabase. 



277 



again occurred forcing up the still unconsolidated portions of the 

 magma through fissures. That part already consolidated may have 

 been warm, and so have permitted of a slow cooling of the second- 

 ary dikes, thus enabling them to take on a granular structure. The 

 mineralogical composition of these dikes, compared with that of the 

 main body, would suggest that either the conditions of cooling were 

 different, or that there was a slight variation in chemical composi- 

 tion of the magma. 



In a canon which cuts across the main body about half a mile 

 from its southern end, one of the largest of the secondary dikes is 

 well exposed. The surface has been blasted off in search for silver, 

 and good specimens for study were obtained. Near the edge of 

 the dike there are irregular cavities, some of them two centimeters 

 in diameter, partly filled with calcite and analcite. The analcite is 

 also disseminated through the body of the rock in water clear 

 grains, some six millimeters in diameter. They break with a 

 splintery fracture and are frequently iridescent. In many cases this 

 mineral is undergoing a change to a radial fibrous aggregate, deter- 

 mined by blowpipe tests to be natrolite. In some of the dikes the 

 latter has almost entirely replaced the analcite. As a rule the feld- 

 spar and pyroxene show no striking peculiarities, although the 

 former has a dull luster indicating decomposition. In the large 

 dike, from which the most of the material was collected, the pyroxene 

 is present in crystals which are sometimes fifteen millimeters long, 

 and show a pronounced lamellar cleavage. These crystals are 

 lustrous black on the outside, but within, a large proportion of them 

 exhibit on part of the cleavage faces a bright silvery luster. The 

 shining appearance is associated with the formation of a transversely 

 fibrous structure. The boundary of this area is sharply defined 

 from the black, lustrous outer portion of the crystal by a straight 

 line of contact parallel to the long diameter of the crystal. What 

 seem to be minute irregular fissures appear in the centers of these 

 cleavage faces and extend the length of the crystals. While the 

 silvery portion has an apparently fibrous structure, this is not exhib- 

 ited when the crystal is split; but, instead, there is a strong tendency 

 to the formation of thin, almost micaceous plates parallel to the 

 diallagic cleavage. 



