ALTERATION OF THE COUNTRY-ROCK. 233 



It would at first glance seem more likely that the rock in the vicinity 

 of the quartz-filled veins would have undergone a silicification. Such is 

 not the case. Instead of a silicification there is, as a rule, a most marked 

 carhonatization, or a conversion of the country-rock to carhonates. ^Fost 

 intense next to the vein, the alteration gradually decreases at a distance 

 from it, the width of the altered zone varying according to the width of 

 the vein. The carhonate zone, surrounding the quartz-vein on ))oth 

 sides, may often ])e studied to great advantage in small veinlets cutting 

 through hanil si)ecimens. 



This action ui)on the adjoining country-rock is in itself, to my mind, 

 the strongest possible evidence against the application of lateral secretion 

 in its narrower sense to tliese veins. It appears to completely refute the 

 theory of the veins being formed by })ercolating surface waters, and i)rove 

 the existence of an agency active in the fissures and gradually extending 

 outward. 



The solutions circulating in the fissures acted with different intensity 

 on difierent rocks. Nearly all igneous rocks, acid or ])asic, are profoundly 

 altered, the latter more than the former, and serpentine more than any 

 other. Only extremely silicious rocks, and especially certain carbona- 

 ceous slates, a})pear to successfully withstand the action of these solu- 

 tions. The process of carhonatization has not in all cases been carried out 

 to its full extent; in some veins it is more marked than in others ; occa- 

 sionally fresh rock may lie close up to the vein.* Crushing of the rock 

 next to the vein facilitates the process and increases the width of tlie 

 altered zone, which may vary from a few inches up to twenty feet, and 

 even more in exceptional cases. With all variations, there is no doubt 

 that the process is a general one, and cliaracteristic for the t^^pcf 



The result of the process, when it has been thoroughly carried out, is 

 the conversion of the country-rock l^y replacement to a mixture of car- 

 honates, white potassium-micas (sericite), a small amount of chloritic 

 minerals and residuary quartz ; besides, there is always a large amount of 

 iron pyrites.^: usually more than in the vein; arsenical pyrites J is also 

 fre(]uently present, Init never, as far as I know, any other su][)hi(les in 

 noticeable amounts. Calcium carl)onate usually prevails, but tlie car- 

 bonates of magnesium, iron and manganese are also present. Accord- 

 ing to numenjus analyses, calcium is always added, while nearly all 

 of the sodium is carried away. The potassium of the orthoclase re- 

 mains transferred to the sericite. As abundant potassic micas are often 



•Such rascH are perhnps due to layers of impervious clay-like detritus on the wall. 



t A )<ood instance has been described by the author in the Fourteenth Ann. Rep. U. S. Geological 

 Survey, in a paper entitled "The (iold-silver Veins of Ophir, California," now in press 



I Both occur HJi small but extremely sharp crystals, while the sulphurets in the quartz are usually 

 massive. 



