OEIGIN AND DEVELOPMENT OF OEE-DEPOSITS. 351 



limited sense the injection theory has a very important 

 hearing upon the origin of ore-deposits, since the 

 injections of elvan and other eruptive rocks appear to 

 have been of vital effect in innumerable instances, 

 especially as regards ores of copper and tin. ■ 



h. — That the fissures have been filled by vapours arising 

 from below. 



There are cases in which this process has undoubtedly been 

 effective — as for instance in the deposition of sulphur, 

 chlorides of iron and copper, specular-iron, and many 

 other minerals in vtjlcanic districts. These deposits, 

 however, are not of the nature of true lodes. Indeed 

 it is hard to see how fissures could remain open for the 

 passage of vapours below the permanent water-level of 

 a county, except very locally and under very exceptional 

 circumstances. If there are in the depths of the earth 

 open spaces in which vapours exist, these could not fail 

 to be absorbed by the waters occupying the fissflres 

 nearer to the earth's surface, thus forming solutions 

 which would come under the next head. 



c. — That the matters in question have been brought into 

 the fissures in a state of solution. 



There are three modifications of this theory, one or more of 

 which are now held, I believe, by almost all who have 

 studied the phenomena of lode-fissures. They may be 

 defined as 



(1). —Infiltration from above (descension theory). 



(2). — Infiltration from the sides (lateral secretion theory). 



(3). — Solutions coming from below (ascension theory). 



All of these modes seem to have been effective, and neither 

 of them in any way excludes either of the others, while the 

 subterranean circulation already discussed and illustrated would 

 supply the necessary active agency for all. Let us examine the 

 question a little more closely by the aid of the following 

 propositions. 



1. All the branches of the " solution and circulation 

 theory," as it may be called, assume the pre-existence somewhere 

 in the path of the circulating waters, of the veinstones and ores 

 now found in the lode-fissures. 



