26 



University of California. 



LVOL. 2. 



planes. One set of areas extinguishes with the unaltered crystal, 

 while the other makes varying angles with it. In some cases this 

 granophyric growth has been from the outer border of the crystal 

 into the angular spaces originally occupied by the analcite. In this 

 case neither set extinguishes with the adjoining crystal of feldspar, 

 but one blends into it as the stage is revolved in the same manner 

 as the basic core of the crystals gives place to the acid rim. 



In many respects the pyroxene of these rocks closely resembles 

 that of the Cuyamas eruptive. In thin section it is a pale brownish 

 green basaltic augite, showing no pleochroism. It has a bright 

 almost metallic luster, and is, in most cases, very fresh. It differs 

 from the augite described by the author in the Cuyamas eruptive in 

 showing no diallagic cleavage, but has the peculiar parting so closely 

 resembling normal twinning. In only one instance was there noticed 

 any alteration, and that was a change to green hornblende. In the 

 panidiomorphic facies the augite occurs as long slender individuals 

 with octagonal cross section (Plate 2, Fig. 3). Terminations con- 

 sisting of the basal plane and orthodome were sometimes observed. 

 The extinction angle is very high, approximating 54 . Many sec- 

 tions were obtained in the different slides cut parallel to the c axis, 

 and a surprisingly large number gave high angles of extinction. 

 Twinning is very common, the composition plane being the ortho- 

 pinacoid. The peculiar parting plane described in the Cuyamas 

 eruptive is frequent. Physically this in every way resembles the 

 composition plane of ordinary twins, but differs in the fact that the 

 two halves always extinguish simultaneously. No reason was found 

 for doubting the conclusion reached before that it might be explained 

 by the supposition that there is here a juxtaposition of two individ- 

 uals similarly oriented. It is quite possible that they may be sepa- 

 rated by a lamellar twin of such thinness as to be indiscernible. A 

 large number of twins were observed in which the difference of 

 extinction of the two halves was 3 to io°. There is a gradual tran- 

 sition from the idiomorphic augites to the ophitic and poikilitic. In 

 the latter the crystals seldom touch each other, but are separated by 

 the greater part of the feldspar and analcite. The poikilitic crystals 

 contain both magnetite and analcite in addition to feldspar. The 

 analcite is not as abundant as in the feldspar matrix between the 

 augites (Plate 2, Fig. 4). 



