Fairbanks.] 



Analcite Diabase. 



283 



consider it as being due to the interposition of an exceedingly thin 

 lamellar twin, in which case the two outer twins would extinguish 

 simultaneously. It could not be considered of secondary origin, 

 for it is fully as prominent in the freshest crystals, seeming to have 

 no connection with a cloudy fibrous appearance in many of them. 

 It is quite possible, however, that it represents the junction of two 

 individuals with the same molecular orientation, and that, instead 

 of ordinary twinning, in which alternate individuals are reversed, 

 in this case the reversion did not take place. An approximately 

 basal section was observed in one slide in which normal twinning 

 appeared, the two halves not extinguishing simultaneously, while 

 in addition one of the halves contained the trace of this peculiar 

 parting. This plane divided it into two slightly unequal portions, 

 both of which showed the same polarization tints and extinguished 

 simultaneously. 



Another remarkable feature of the augite, quite distinct from 

 the last, is the very perfect diallagic cleavage and the silvery lustered 

 interior portion. In thin section the latter is indicated by fibrous 

 cloudy areas, somewhat irregular in shape and generally confined 

 to the central portion of the crystal. Plate 15, Fig. 3, is a good 

 illustration of this decomposition. A cleavage plate parallel to the 

 orthopinacoidal parting was obtained, showing the nature of the 

 alteration. The fibers lie transverse to the c axis, fading out toward 

 the periphery, their extremities being terminated by the traces of 

 the planes of the prismatic cleavage. (Plate 15, Fig. 4.) In the hand 

 specimen this cleavage is very strongly pronounced, and, judging 

 from the manner in which the crystals split, it is not confined to 

 one plane, but may be developed in any portion of the crystal. 

 The perfect cleavage and silvery luster of the augite individuals in 

 the hand specimen is one of the most striking features of the rock. 

 This phenomenon undoubtedly represents the beginning of a change 

 to diallage, but, instead of starting from the outside, as is usually 

 the case, the alteration has commenced at the center, and the fibra- 

 tion has taken place transversely instead of longitudinally. 



Analcite. — The first thing in the hand specimen that attracts 

 one's attention is the presence of clear, glassy grains, with a faint 

 iridescent luster. Under the microscope they are found to be 



