1850.] HOPKINS CRYSTALLINE ROCKS OF THE ANDES. 365 



Many thousand square miles of this vertical structure may be seen 

 in South America, and in magnificent sections several thousand feet 

 deep, exhibiting a great variety of colours, crystallization, and trans- 

 itions of the primary series ; — as may be observed on reference to the 

 sections exhibited to the Society, which have been carefully made 

 from a very laborious survey carried on during many years, geologi- 

 cally, mineralogically, barometrically, and trigonometrically. This 

 survey was not alone confined to the surface, but it comprehends the 

 results of very extensive subterranean explorations in various parts of 

 the Andes. 



One section, not published with this paper, intersects the three 

 branches of the Cordilleras in latitude 5° 10' N, ; from the Pa- 

 ramo of Chingasa to the head of the river Atrato, and passes through 

 the silver-mines of Mariquita and the gold-mines of Supia, and many 

 other mining localities of great interest. 



The accompanying section (Plate XXXI.) crosses the central Andes 

 in latitude 4° 20' N., from the plains of the river Meta (a tributary 

 of the Orinoco) to the shores of the Pacific Ocean at Choco ; being 

 about 260 geographical miles from E. to W. It exhibits at one 

 glance a great variety of rocks, crystalline and sedimentary, including 

 also the gold regions. 



Another section has been made which passes through the emerald 

 mines of Muzo, the various marbles and granites near Nace, and 

 other important rocks, on the banks of the Rio Negro in Antioquia, 

 in latitude 7° N. The accompanying section, however, will, I trust, 

 serve the purpose of this paper. It would occupy too much time to 

 enter into the details of all the rocks, their uniform vertical struc- 

 ture, and their respective transitions ; therefore I shall simply state 

 in general terms that the great Cordilleras are formed of innumerable 

 varieties of granites, gneiss, schists, hornblende, chloritic slates, por- 

 phyries, &c., as noted in the section, and these rocks alternate with 

 each other in great meridional bands ; which in the ridges frequently 

 present the appearance of a radiated or fan-shaped structure, and 

 under the plains are more or less vertical. The bearing of the 

 cleavage-planes of the structure is quite independent of the direction 

 of the ridges, secondary chains, or the sinuosity of the valleys : — they 

 pass for hundreds, indeed I may say thousands, of miles in a geome- 

 trical order, through all the superficial undulations without deviation, 

 on an average bearing of about 30° N.E. This uniformity of bearing 

 causes the prevalence of this strike in the older sedimentary rocks, 

 which have been altered, and cleaved in the direction of the primary 

 structure, as seen in Wales, and in the roofing-slate districts of the 

 Continent. 



On reference to the section it will be seen that the crystalline rocks 

 follow no particular order in the alternation, as commonly supposed ; 

 on the contrary, we find only granites and gneiss for miles ; and again 

 schist, quartz, gneiss, &c. interchanging. 



However, there is one general order in the compound both verti- 

 cally and longitudinally, viz. each variety of schist depends on its 

 crystalline parent below, that is, on the quality of the granitic base. 



2 D 2 



