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University of California. 



[Vol. 2. 



dike, it is apparently as abundant as in the Cuyamas eruptive. In 

 the poikilitic facies, however, where the feldspars are very small, 

 the angular areas occupied by the analcite are correspondingly small, 

 and it is not visible to the unaided eye. There are large areas of 

 the rock also where it has been replaced by decomposition products. 

 The microscopic examination shows that in the coarser rocks it has 

 undergone but little alteration. Greenish products appear, as well 

 as those referable to natrolite. The latter forms cloudy radial 

 aggregates. The finer-grained rocks probably contain fully as much 

 analcite, but it is often disseminated in such small areas that it is 

 not at first noticeable. These smaller areas have undergone altera- 

 tion more easily, and, as a consequence, fresh analcite is rare. 

 Natrolite is perhaps its most common alteration product. It occurs 

 associated with the feldspar within the poikilitic crystals of augite, 

 but is particularly abundant in the feldspar aggregates between 

 them. Its abundance in comparison with the feldspar is somewhat 

 less within the augites than between them, where it often forms 

 nearly one-fourth of the area. A dozen small polyhedral areas of 

 analcite or natrolite were counted within one augite individual, 

 occurring in the same manner as in other portions of the rock, that 

 is, filling spaces probably once occupied by a residual portion of the 

 magma (Plate 2, Fig. 4). These areas are frequently in contact 

 with the augite, being bounded in part by that mineral and in 

 part by the feldspar. It seems to the author that the presence of 

 the analcite, showing this relation to the large augite individuals, is 

 of the greatest importance in aiding us to reach a true conception 

 of the original condition of this peculiar rock. Not only is it a 

 fact that an original miarolitic structure has never been seen in 

 rocks of the diabase class, but it is improbable that the spaces within 

 the augites should ever have been left as empty cavities. These 

 areas, as cut by the section, are wholly inclosed in the feldspars, or 

 are scattered in bunches through the augite, or are bounded by both 

 the augite and the feldspar. The bounding plane of the augite in 

 the last case always appears to be a crystal face, and has that regu- 

 larity which would be expected if the area now filled with analcite 

 were at the time of crystallization filled with an unconsolidated 

 residual portion of the magma. One instance was noted in which 



