474 NOTES ON SOME OXIDISED COPPER ORES. 



was sunk some few years ago to a depth of 150 feet through 

 gravels and sands. On reaching this depth a sort of " hard-pan " 

 was broken through and the water immediately rose to within a 

 short distance from the surface — where it now remains nearly 

 constant at all seasons of the year, although the annual rainfall 

 is very variable, and in some seasons no rain falls for many 

 months together. In other places, however, nearer the hills, the 

 detritus is very much thinner or even altogether absent, and here 

 the strata are seen to consist of nearly horizontal beds of lime- 

 stone, precisely similar to that which forms the eastern border of 

 the plain in this neighbourhood, and which has been described 

 as Jurassic by the Grovernment geologists from the city of Mexico. 

 In this limestone certain fissures have been found containing 

 bones of the horse, deer, and ox, together with those of smaller 

 quadrupeds and of many birds. Up to the present I believe 

 these bones have not been reported upon by any competent 

 palaeontologist. The hills referred to rise to a height of from 

 600 to 1200 feet above the plain. Some are composed of the 

 limestone already referred to but tilted up at considerable angles, 

 apparently by intrusions of light-coloured felspar, porphyry, and 

 felsite. Others again are composed of masses of this sahie 

 porphyry. Of the up-tilted limestones the hill known as the 

 Cerro de la Cruz is a good example, while the neighbouring 

 Cerro de Magistral is a fine example of the larger felsitic intru- 

 sions. Both limestone and porphyry have been disturbed by 

 subsequent intrusions of garnet-rock, which is in some places, 

 and especially on the hill known as El Cumbre, almost stratified 

 in appearance and inclined eastward at a high angle. 



Mode of occurrence of the Ore-deposits. — AU the hills in the 

 neighbourhood of Terrazas station seem to be more or less per- 

 meated with metallic minerals, ores of lead, copper, and iron, 

 all more or less argentiferous, and some very highly so, 

 being found in each of the rocks mentioned, while occasional 

 pockets of true silver ores of great richness have been met with 

 from time to time. The copper-ore occurs in all the rocks, but 

 mostly in close proximity to the garnet-rock intrusions. The 

 porphyry is often stained with copper in various forms, and 

 occasionally distinct ore masses occur in it, but only near the 

 garnet, and as a rule it is not highly cupriferous. The limestone 



