102 IOWA ACADEMY OF SCIENCES. 



In the lon^ list of published analyses of the crystalline 

 rocks, the feature most striking to the student of ore depos- 

 its is the apparent absence of metallic elements. Several 

 factors contribute to this apparent anomaly. In the first 

 place the majority of rock analyses have been made pri- 

 marily to establish the relationships of the non-metallic 

 elements composing the rock forming materials. The per- 

 centage sum of the minerals leaves a very small margin for 

 the minerals which from a strictly metallurgical stand- 

 point are important. With the exception of iron and one 

 or two other metals, no account is taken of the metallic 

 elements which may be present. 



The fact is well illustrated by the Missouri rocks. The 

 Graniteville granite, when analyzed for petrographical pur- 

 poses showed no metals other than iron and maganese. 

 When tested by more refined methods, the identical rock 

 was found to contain 0.00126 per cent of lead, 0.00216 per 

 cent of zinc and 0.00176 per cent of copper. 



Owing to the usual methods of analysis these, if they be 

 present, are calculated as iron or some other metal. 

 This is sufficiently close for the immediate purpose of 

 petrography. It is in fact all that is demanded of the rock 

 student. Analysis more refined is entirely unnecessary in 

 rock investigations. 



It is a fact that has attracted wide attention that the 

 results of the more recent rock analyses present a very 

 different aspect from that of the earlier and majority. 

 Some of these have been undertaken with the express pur- 

 pose of determining those metallic elements which occur 

 in very minute quantities; others have been made with 

 the view of getting more exact statements for special fea- 

 tures. In nearly all of these the presence of several of the 

 rarer metallic compounds is a feature that is quite note- 

 worthy. 



An exhaustive critical review at this time, of what has 

 been done in regard to this phase of the subject, would be 

 out of place. The comparatively few examinations which 

 have been made with the special object in view of deter- 

 mining the character of the metallic contents of rocks have 



