Waring — Lavas of Morro Hill a^id Vicinity. ' 101 



granitic basement rock, for granite and diorite are exposed 

 completely around both hills, as is indicated in fig. 2, in which 

 the unruled areas are of the ancient crystalline rocks. 



The eastern slope of the hill, although steep, may be as- 

 cended without great effort. On the west face there is a cliff 

 50 feet or more in height, as is seen in figs. 3 and 4. At one 

 place in the top of this cliff the rock is disintegrated and has 

 been markedly lioneycombed by the wind. 



In the main hill and the subsidiary one adjacent to the 

 southeast there are at least three classes of material, whose ap- 

 proximate areal extents are indicated in fig. 2. The rock of 

 the main hill, locality 1 {^g. 2), is a fine-grained, gray andesite. 

 A slide of the rock shows a groundmass composed principally 

 of felsite with some glass and orthoclase. The feldspars in 

 places show fiow structure. Some magnetite and carbonaceous 

 material are also present. 



On the north side of the hill, nearly half-way up its slope, 

 at locality 2, a small cliff of coarse-grained dioritic rock is ex- 

 posed. A slide shows it to be composed principally of plag- 

 ioclase and orthoclase with some quartz, magnetite and 

 hematite. The rock appears to have been considerably altered, 

 as the ferro-magnesian minerals have been entirely oxidized, 

 leaving only the shapes of the former hornblende crj^stals. 



On the south side of the hill at locality 3, between the lava 

 and the underlying diorite, is a rock composed of grains of 

 dioritic material. A slide shows it to be composed principally 

 of plagioclase and orthoclase with quartz, magnetite and 

 hematite. The rock is much finer grained than that of local- 

 ity 2, on the north side of the hill, and the minerals are con- 

 siderably broken up into grains, as if by water action. It 

 seems probable that the andesitic intrusion took place after a 

 period of weathering of the diorite and that this rock which 

 immediately underlies the andesite consists of the baked, de- 

 composed diorite. The absence of ferro-magnesian minerals 

 would bear out this supposition. 



The lower hill, locality 4, immediately southeast of Morro 

 Hill, is composed of andesite which is a blend of that exposed 

 at locality 1, with small fragments of the dioritic rock or its 

 minerals, through which it has been intruded, and also small 

 fragments of pink tuff. A slide of the rock exhibits a felsitic 

 groundmass with considerable glass and carbonaceous material, 

 and occasional corroded crystals of plagioclase and orthoclase. 

 Green epidote forms patches throughout the groundmass. 

 On the north and w^est the limit of the lava is marked by steep 

 slopes and small landslide scars, where the material has disin- 

 tegrated to a deep black adobe immediately overlying the 

 granite. Along the eastern and southern borders of the hill 



