62 BULLETIN OF THE ESSEX INSTITUTE. 



The structure of the rock Jilso differs from that of the 

 Pickard's Point dike, which has the typical tinguaitic 

 structure, in that the component minerals do not occur 

 in needle forms, but in much stouter lath-shapes, show- 

 ing a greater degree of crystallization for the individual 

 minerals, and producing a much less dense phase of tin- 

 guaite. The presence of many plates of feldspar tabular 

 to M indicates an approach to a solvsbergite, and the rock 

 might perhaps with equal right be considered a phase of 

 a nepheline solvsbergite. It seems in structure and com- 

 position to lie intermediate between a nepheline tinguaite 

 and a nepheline-aegirine-solvsbergite. 



The analysis of the rock yields 



100.04 



The spooitio gravity determined l)y the balance is 

 2.708. The dike is difficult to reach and the specimens 

 examined come from near the surface and have altered 

 enough to make it difficult to estimate the mineral con- 

 tents with any degree of accuracy. It is at once appar- 

 ent that the percentage of alkalies is too low to use up 



