22 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



the characteristic lozenge-shaped cross-sections were seen, 

 with a distinct prismatic cleavage. Twinning is common, 

 parallel to the orthopinacoid. Comparatively few unaltered 

 sections were found, most of the hornblende which was 

 originally present in the slides having been altered to chlo- 

 rite and calcite. In some of the slides the hornblende is 

 wholly replaced by secondary minerals. The freshest sec- 

 tions are frequently surrounded by a chloritic border, or 

 decomposition has begun along cracks and cleavage planes. 

 The pleochroism of both chlorite and hornblende is the 

 same as in the diorites. The hornblende in some cases has 

 the fibrous character described under the diorites. In one 

 slide in which this fibrous hornblende occurs, without good 

 crystal boundaries, there is another secondary hornblende, 

 with different optical properties, and having the form char- 

 acteristic of augite. 



From what has been said it will be seen that hornblende 

 is a primary constituent of the rock. It is also the dom- 

 inant ferromagnesian mineral. Roughly estimated, it con- 

 stitutes, with its decomposition products, about one-fourth 

 or one-third of the total amount of minerals in the slides. 



Augite is found in varying amounts in nearly all the slides, 

 usually in the form of irregular grains, or as irregular 

 brownish or more or less opaque patches. It has the same 

 general structure and habit as in the diorite and its decom- 

 position products are similar. Usually it has a very insig- 

 nificant position compared with the other minerals of the 

 rock. In two or three of the slides, however, it compares 

 in amount with the hornblende, and in one slide in particu- 

 lar — from a specimen from the canon back of Avalon — the 

 sections, though rather small, are quite numerous, constituting 

 perhaps one-fifth or one-fourth of the total minerals of the 

 slide. Here the mineral is fairly fresh and free from decom- 

 position products. 



It is almost colorless or pale green, and without pleochro- 

 ism. When any crystal boundaries are shown they are 

 only partial. The form when developed shows the usual 

 octagonal cross-section. The crystals occur as separate 



