430 La Plata : part n. 



age, generally ranging east and west. E. and SE. of 

 Las Minas there is a confused district of imperfect 

 gneiss and laminated quartz, with the hills ranging in 

 various directions, but with each separate hill generally 

 running in the same line with the folia of the rocks of 

 which it is composed : this confusion appears to have 

 been caused by the intersection of the [E. and W.] and 

 [NNE. and SSW.] strikes. Northward of Las Minas, 

 the more regular northerly ranges predominate : from 

 this place to near Polanco, we meet with the coarse- 

 grained mixture of quartz and feldspar, often with the 

 imperfect hornblende, and then becoming foliated in 

 a N. and S. line — with imperfect clay-slate, including 

 lamiD&3 of red crystallised feldspar — with white or black 

 marble, sometimes containing asbestus and crystals of 

 gypsum — with quartz-rock — with syenite — and lastly, 

 with much granite. The marble and granite alternate 

 repeatedly in apparently vertical masses : some miles 

 northward of the Polanco, a wide district is said to be 

 entirely composed of marble. It is remarkable, how 

 rare mica is in the whole range of country north and 

 westward of Maldonado. Throughout this district, the 

 cleavage of the clay-slate and marble — the foliation of 

 the gneiss and the quartz— the stratification or alter- 

 natiug masses of these several rocks — and the range 

 of the hills, all coincide in direction ; and although the 

 country is only hilly, the planes of division are almost 

 everywhere very highly inclined or vertical. 



Some ancient submarine volcanic rocks are worth 

 mentioning, from their rarity on this eastern side of 

 the continent. In the valley of the Tapas (fifty or 

 sixty miles N. of Maldonado) there is a tract three 

 or four miles in length, composed of various trappean 

 rocks with glassy feldspar — of apparently metamorphosed 

 grit-stones — of purplish amygdaloids with large kernels 



