152 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 23, 



tirely replaces the other metalliferous minerals, the proportion of 

 iron gradually preponderating on going down. 



4. The horses, or areas of unproductive ground occurring within 

 the lode and between courses of ore, consist for the most part of 

 porphyry, identical in appearance with the rock outside the lode, 

 but generally mineralized with iron and copper pyrites. 



5. The metalliferous deposit, obeying the form of the ground, 

 terminates abruptly to the west, where the hill is precipitous, and 

 dies away towards the east. The heaves and cross courses do not 

 carry ore. 



The Santiago Lode. — Following the main lode from its chief deve- 

 lopment towards the east, its outcrop may be traced at intervals 

 until we reach a point where a contra lode or main branch forms a 

 junction with it. This junction is well seen in a natural section 

 formed by the river-banks, and beyond it, towards the east, the out- 

 crop continues remarkably strong, and possesses many points of in- 

 terest. The contra lode or branch makes an angle of 30° with the 

 main lode, and goes away to the south-west. It is seen at various 

 points, and after about half a mile bifurcates, and is crossed by 

 several small strings and subsidiary lodes, traceable in various direc- 

 tions, and not yet proved to be connected with each other in any 

 important sense. Some of these strings and lodes are of considerable 

 size (measuring underground from three to seven feet) and have 

 shown, either at the surface or at some depth, very good coppery 

 indications, consisting of oxides, carbonates, and rich sulphurets of 

 copper. The gossans of such lodes have generally led down to 

 bunches of unusually rich sulphuret, but no steady and continuous 

 deposit has yet been proved to exist, resembling that found in the 

 Cobre concessions. The country also is here less metamorphosed, 

 calcareous green conglomerates replacing the porphyries, while 

 elvans of porphyry occur amongst the lodes. Generally speaking, 

 in this part of the mineral field the appearance of the lodes when 

 cut underground has not been so satisfactory as might have been 

 expected from the character of the gossans, and whilst the ore 

 obtained has been of the finest quality, the quantity has been too 

 small, and the supply too irregular, to secure a profit on the mining 

 operations carried on. 



Besides the group of lodes cropping out to the south of the Cobre, 

 and connected with the Santiago lode, trials have been made on 

 small gossany outcrops in the valley of the Cobre, to the north of the 

 lode, and also in the ground to the west. Bunches of ore exist under 

 these crops, but no valuable deposit of copper has been found. 



Metamorphism of the enclosing rock. — Having thus described the 

 circumstances of the Cobre lode, and its principal contra lode, it 

 remains for me to allude to one very important fact regarding the 

 enclosing and associated rocks. I have already described these as 

 consisting of grits and conglomerates, passing into metaraorphic rocks, 

 which include porphyritic conglomerates, greenstone, and even basalt. 

 The circumstances under which these changes take place are too 

 remarkable to be passed over. 



