Fairbanks. 



A nalcite Diabase. 



grew as druses in originally empty spaces in the rock. This would 

 of course affect the question of the original presence of nepheline. 

 In the first place it may be said that the condition of occurrence of 

 several of the minerals of this rock including the prehnite is quite- 

 peculiar. It seems to the writer probable that the prehnite was 

 formed either during the alteration of the nepheline, from which the 

 analcite is supposed to have been derived, or directly from the anal- 

 cite itself. In many of the feldspar crystals the alteration to analcite 

 has left as sharply defined rectilinear lines as are shown to exist 

 between the analcite and the prehnite, but in the former case there 

 is no tendency to the formation of crystal boundaries. The writer 

 does not deny that there may have been open spaces left in portions 

 of the rock mass at the time of solidification; the presence of anal- 

 cite partly filling some of these in the largest secondary dike sup- 

 ports that view. 



The discussion of the question concerning the original presence 

 of nepheline has been given in another place. In addition it might 

 be said that the existence of a similar structure (the polyhedral 

 areas filled with analcite) in all the known bodies of rock of this 

 type, and in this type only, in the Coast Ranges — bodies of varied 

 size, which must have solidified under different conditions — is incon- 

 ceivable unless we postulate some original inherent peculiarity of 

 composition. This is best illustrated in the case of the Cuyamas 

 eruptive. The polyhedral areas filled with analcite are found to 

 occur in a similar manner in the large primary mass, which is a 

 thousand feet across, as in the numerous secondary dikes from four 

 inches to twenty feet wide. It is hardly possible that similar condi- 

 tions of solidification could have existed in bodies of such greatlv 

 diverse size. While the writer does not hold that the original pres- 

 ence of nepheline can be demonstrated (unless in the case of one 

 dike in which hexagonal areas of analcite occur), yet, taking every- 

 thing into consideration, there seem to be fewer difficulties opposed 

 to that view than to any other. 



The other mineral, supposed to be feldspar, is found bordering 

 the analcite and growing inward. It generally occurs as an irregu- 

 lar border of microscopic proportions. Sometimes the outer edge 

 is wavy (Plate 16, Fig. 3), at others rectangularly serrated. It is 



