CHILL 579 
fresh specimens. In this decomposition, it may be observed that the 
feldspar first yields, becoming white and opaque, with a friable earthy 
appearance. ‘The mica also, if black, is soon changed in colour to a 
rusty brown, owing to the development of the iron it contains, and it 
loses at the same time its elasticity and cohesion. The syenitic and 
hornblendic rocks of the region are more durable than the gneissoid 
granite. Mellaca Hill consists on one side of a decomposable granite, 
and on the opposite of a durable syenite. ‘The rock on the ridge 
above Valparaiso is a granite containing very little mica. It is 
hastened in its alteration, or disintegrated, by the oxydation of mag- 
netic iron, which is sparingly disseminated in grains or small veins. 
The feldspar shows little tendency to decomposition until minutely 
divided by the iron. ‘The rock crumbles into a fine gravel, which has 
a red tinge from the oxyd of iron. 
GREENSTONE, BASALTIC AND PORPHYRITIC ROCKS OF CHILI. 
In the following account of these rocks, their modes of occurrence, 
characters and transitions are mentioned as they appeared along 
different routes pursued by us: we had no time for much exploration. 
A league to the north of Valparaiso, on the road which follows the 
coast, two dikes of greenstone intersect the gneissoid granite. The 
greenstone in one of these dikes presents scarcely any traces of crys- 
tallization, and consequently no signs of its constituent minerals. It 
is soft and has a glistening lustre. ‘The rock of the other dike, a few 
rods beyond, is similar to the first, but is more granular in texture, 
and contains light-green particles of some decomposed mineral, which 
appeared to be feldspar. It breaks with an earthy fracture. Nume- 
rous dikes in the Cuesta de Zapata, on the road to Santiago, afford 
the same variety of rock, and it is also abundant in the Cuesta de 
Prado. 
This earthy greenstone often passes into a more compact rock, 
having the hardness of feldspar and an impalpable structure, as is 
observed in the Cuesta de Prado, where much of the greenstone is 
tough and compact. In the mountain back of Santiago we found a 
similar compact greenstone; and also a porphyritic variety containing 
small crystals of feldspar. 
Mellaca Hill, at Quillota, in the valley of the Concon, is intersected 
