chap. xiii. Crystalline Rocks of, 431 



of carbonate of lime ] — and much of a harshish rock 

 with glassy feldspar intermediate in character between 

 claystone porphyry and trachyte. This latter rock was 

 in one spot remarkable from being full of drusy cavities, 

 lined with quartz crystals, and arranged in planes, 

 dipping at an angle of 50° to the east, and striking 

 parallel to the foliation of an adjoining hill composed 

 of the common mixture of quartz, feldspar, and im- 

 perfect hornblende : this fact perhaps indicates that 

 these volcanic rocks have been metamorphosed, and 

 their constituent parts re-arranged, at the same time 

 and according to the same laws, with the granitic and 

 metamorphic formations of this whole region. In the 

 valley of the Marmaraya, a few miles south of the 

 Tapas, a band of trappean and amygdaloidal rock is 

 interposed between a hill of granite and an extensive 

 surrounding formation of red conglomerate, which (like 

 that at the foot of the S. Animas) has its basis porphy- 

 ritic with crystals of feldspar, and which hence has 

 certainly suffered metamorphosis. 



Monte Video. — The rocks here consist -of several 

 varieties of gneiss, with the feldspar often yellowish, 

 granular and imperfectly crystallised, alternating with, 

 and passing insensibly into, beds, from a few yards to 

 nearly a mile in thickness, of fine or coarse grained, 

 dark-green hornblendic ^late ; this again often passing 

 into chloritic schist. These passages seem chiefly due 

 to changes in the mica, and its replacement by other 

 minerals. At Rat Island I examined a mass of chloritic 

 schist, only a few yards square, irregularly surrounded 

 on all sides by the gneiss, and intricately penetrated 

 by many curvilinear veins of quartz, which gradually 

 blend into the gneiss : the cleavage of the chloritic 



1 Near the Pan cle Azucar there is some greenish porphyry, in one 

 place amygdaloidal with agate. 



