34 



University of California. 



[Vol. 2. 



large percentage of soda in a rock containing a lime-soda feldspar. 

 But for the excess of soda, the composition of the rock and its 

 structure would, of course, place it with the diabases. In the case 

 of the rock under investigation, the analcite must have one of two 

 sources: it either represents a primary soda mineral, or it has been 

 introduced into the rock subsequent to solidification. If the latter 

 is the case, the structure must have been originally miarolitic, for 

 the areas occupied by the analcite are bounded by crystals with idio- 

 morphic faces. For the purpose of realizing what this supposition 

 involves, the following considerations will be of value. In the first 

 place, the miarolitic structure has never been observed in the dia- 

 bases or related rocks, and if we except the presence of the analcite, 

 it must be acknowledged that this rock is a typical diabase. The 

 conditions under which a similar structure, possibly miarolitic, 

 occurs both at Point Sal and in the Cuyamas region, are exceed- 

 ingly varied, and it is impossible to believe from what we know of 

 the relation existing between structure and circumstances of cool- 

 ing magmas that it would be the same in all cases. In the case of 

 the Cuyamas eruptive, there is one large mass, 1,000 feet across, 

 cut by numerous dikes, some no wider than six inches, all of which 

 show the same angular areas filled with analcite. At Point Sal 

 there is a greater variation still. The angular spaces filled with 

 analcite and its decomposition products are to be seen in all the 

 facies from the coarse panidiomorphic through the ophitic to the 

 fine-grained aphanitic edges. Dikes two to three inches across of a 

 rock approximately panidiomorphic contain analcite under the 

 same conditions as the inclosing rock, which is ophitic. In addition 

 the latter facies contains analcite, occupying angular spaces within 

 the large augite crystals under circumstances which certainly pre- 

 clude the supposition that they are miarolitic cavities. It is impossi- 

 ble to believe that empty polyhedral areas, not having the form of 

 negative crystals, should have remained withfn the augite during 

 its segregation and crystallization, even though we allow the possi- 

 bility of such miarolitic spaces in the body of the rock. In the 

 case of one of the Cuyamas dikes there occur clearly defined hex- 

 agonal areas which are filled with analcite. There is no possibility 

 of their being referable to apatite, and they are explainable only on 



