PKKSI])KXTIAL ADJJRESS SECTION ii. 47 



At tliis point it seems desirable to emphasise the 

 necessity of caution in applj-ino- current theoretical con- 

 ceptions to particular occurrences which may be encountered, 

 for theories which are widely held to-day may become 

 discredited within a surprisingly short time as knowledge 

 advances. As an instance of how detailed investigation of 

 the facts may alter the views held as to the genesis of certain 

 ore deposits, I may cite the case of some of our Rhodesian 

 gold reefs. The theory that I myself formed, after making 

 a preliminary examination of several mining districts, was 

 that they were intimately connected with the intrusion of 

 the great granite masses which are so prominent a featiu'e 

 of Rhodesian geology, though I pointed out that there were 

 some which appeared to be of earlier and some of later date. 

 Several years after, Mr. Maufe drew attention to the close 

 association of many reefs with finer grained acid intrusive 

 rocks, though he did not clearly indicate whether they were 

 all of one age. It is quite probable that I had not allowed 

 enough weight to the influence of the smaller intrusions, 

 some of which certainly show every evidence of being- 

 genetically connected with the deposition of gold ores. That 

 it does not do, however, to dogmatise too freely on apparent 

 associations is well illustrated by what is now known of the 

 Sebakwe district. The reefs there include those of the largest 

 gold producer in the country, namely, the Cxlobe and Phoenix, 

 as well as the Gaika, Moss, and other smaller properties. These 

 are all situated along the margin of the granite, and might 

 well be claimed as typical examples of the theory of close 

 association with the intrusion of that rock. Closer examina- 

 tion, however, shows that the reefs cut across certain dykes 

 of the granophyre family, which mav be termed felsites, or 

 quartz-jjorphyries. The latter are clearly younger than the 

 granite, and it might, therefore, seem that the deposits belong 

 to the groujj of which the importance has been emphasised 

 by Mr. Maufe. The fierce light of publicity which shone 

 upon the Globe and Phoenix Mine during the great lawsuit 

 over the right to work under the John Bull claims has never- 

 theless had as one result the demonstration that there are 

 still younger igneous rocks which have to be taken into 

 account. These are a series of dykes ranging in composition 

 between dolorite and porphyrite, and usually much altered 

 with production of carbonates, though not affected by the 

 more intense agencies of metamorphism. These were first 

 noticed at the Gaika Mine, and later on in the Eennie-Tailyour 

 Concession, and I had been puzzled by the fact that they 

 occasionally carried gold. The observations at the Globe and 

 Phoenix showed that they cut both granite and quartz por- 

 phyries, but were nevertheless intersected by the reefs in that 

 mine. We thus see how more and more detailed knowledge 

 may gradually alter our opinions regarding the genesis of 

 particular ore deposits, though each one appeared in turn to 

 fit in admirably with the facts as far as they were established 

 at the time. It may be noted that association with the basic 

 dykes seems clearly indicated in this instance as the final 



