562 CH itt. 
soid, and in some places passes to a perfect gneiss, or even a mica 
slate. Dark gray is the prevailing colour. The rock consists of white 
or grayish-white feldspar, white or colourless quartz, and black mica 
in small scales. ‘The mica has occasionally a greenish tinge, and 
sometimes (as half a mile southeast of Valparaiso, where the rock is 
remarkably micaceous,) it has a bright gold-yellow colour. ‘These 
changes are frequent and without regularity... The passage of the 
granite to gneiss often takes place in spots subordinate to the general 
mass of granite. ‘The rock is often a true gneiss at.an exposed point, 
having a distinct dip and direction ; but the same rock, a few feet dis- 
tant, defies all attempts to distinguish the angle of inclination ; and as 
far in another direction it may have all the characters of a true 
eranite.* This is the common fact about Valparaiso. 
A mile to the southwest of Valparaiso, towards the lighthouse, the 
granite becomes syenitic ; at first, there is a sparing dissemination of 
hornblende ; and then the proportion increases till the rock is gneissoid 
syenite and hornblende schist. ‘The syenitic granite has a greenish 
tinge, and the schist a black or greenish-black colour. A similar pas- 
sage of granite to syenite occurs a mile to the north of Valparaiso be- 
yond Essex Beach; also of still greater interest, in the cliff just north 
of the beach of Vina del Mar, three leagues from Valparaiso. The 
granite at the latter place is rather feldspathic, and the mica it con- 
tains, is in coarse black scales. In a neighbouring cliff there are 
planes of fracture which dip to the southwest-by-south forty-five 
degrees; while in that just referred to, there are narrow interpolations 
of a dark gray rock, resembling dikes, and dipping to the southwest 
Fig. 1. Fig. 2. 
seventy degrees. Figure 1 represents a breadth of eighty feet, and 
figure 2 of thirty. This dark rock is between gneiss and mica slate 
* Mr. Darwin alludes to this irregularity, but concluded that the strike of foliation was 
about north-by-west and south-by-east.—Greological Observations on South America, 
p. 162. 
