CHILI. 581 
tough : it is sparingly porphyritic with small crystals of feldspar, and 
contains a little epidote. 
On the descent to the northeastward, over the lower part, the rock 
becomes a gray porphyry ; it is a compact rock, nearly impalpable in 
structure, having a grayish-blue colour and containing large crystals 
of feldspar. These crystals are half an inch or more in length and 
are very thickly disseminated; they are generally compound. The 
rock still contains a little epidote. 
These transitions, here indicated, are a caution with reference to 
inferring a difference of age in the dikes of a region from their litho- 
logical differences; for here we have a number of striking varieties 
in a single ejected mass. 
It appears that more or less epidote is disseminated throughout the 
whole; and where the epidote is most abundant the red colour of the 
rock was deepest. ‘This corresponds with the fact that the epidotic 
seams intersecting granite veins occasioned a red colour in the feld- 
spar adjoining them. The effect is undoubtedly due to iron. It is 
probable that where the rock of the cuesta, when it was in fusion, 
contained too much iron to be all taken up by the epidete, a portion 
remained free as an oxyd, and gave the red tint. Oxyd of iron and 
some lime are all that are needed, in addition to the elements of feld- 
spar, for producing epidote. 
The “first cuesta’” between Quillota and San Felipe consists of 
trachyte, a white feldspathic rock having little lustre and a rough 
fracture. It has usually a slight tinge of red, and is traversed by 
numerous reddish lines. ‘The rock is fragile, and in some places 
fragments cover the surface of the ridge. 
Towards the top of the ridge, the colour becomes a dirty gray. 
At the top it passes to a dark green colour, and becomes porphyritic, 
and the rock then resembles some of the porphyritic greenstones of other 
locahties. 
The predominant igneous rock near the outlet of the Jaguel Valley 
in the Andes, east of San Felipe, is a gray clinkstone (or graystone), 
consisting apparently almost solely of feldspar. ‘The more compact 
varieties have a fine texture without any disseminated crystals, and 
break with a smooth surface and a conchoidal fracture. It passes by 
gradual transitions to a gray porphyry, containing numerous small 
crystals of feldspar or albite ; which of the two, was not satisfactorily 
determined. ‘The colour varies little, except in becoming darker in 
some places. ‘The texture of the rock is at times imperfectly granular. 
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