THE SOUTHERN COMPLEX. 115 



points, and the relations of the mica and })yr()xene to the feldspar are the 

 same as those of the hornblende and biotite in the mica-hornl)lende-sve- 

 nites. This rock also carries some hornblende, but in subordinate' (|llantit^■. 

 In some cases the biotite seems to \)u. forming as a secondary product from 

 it. The pyroxene is colorless, strongly doubly i-efracting-, and from its ()[)- 

 tical properties is taken to be malacolite. It is generally in romidish gran- 

 ules in cross-section, and in blades several times longer than broad in lon- 

 gitudinal section. Many of the biotite blades are much distorted. \\\ of 

 the syenites contain very numerous, large, well formed crystals of apatite, 

 which are included in all of the other minerals. 



Not very far in basicity from these syenites are coarse, altered gabl)ros, 

 which in large exposures are closely associated with them. In the field 

 and in hand specimen they do not differ greatly in appearance from some 

 of the syenites just described. Their feldsi)ars have a reddish cast, as in 

 them, and their structure is nuicli the same. However, when they are 

 examined in thin section they are seen at once to be altered basic eruptive 

 rocks of the oi'dinary type. The chief original constitu(aits are plagio- 

 clase, magnetite, anddiallage, which have relations characteristic in a rock in- 

 termediate in structure between a diabase and a gal )bro. The })lagioclase has 

 altered very extensively to chlorite and kaolin, and is in many places re[)laced 

 to some extent by saturating quartz. The magnetite occm's in well defined 

 areas and crystals, which give no evidence of alteration, it being the onh' min- 

 eral within the rock which is unaffected. The pyi'oxene has very largely 

 decomposed, the resultant products being hornblende, l)iotite, and chlorite, 

 more largely the first. It is a very noticeable thing that the secoiidin-\' horn- 

 blende is not paramorphic but several or many blades have foi-metl whicli 

 are in crystallograjihic i)osition independent of the original augites, and 

 much of this hornl)lende in its forms is like the hornblende contained in the 

 syenites just described. As a result of the complete decomposition of the 

 diallage, hornblende and biotite are formed with their a.xes arranged quite 

 often in a common direction. In this case these minerals, in their appear- 

 ance and relations, are much like those of the same minerals in the associ- 

 ated syenites. It is further noticeable that these rocks also coiitain numer- 

 ous large, well formed crystals of apatite. Considering all of the foregoing 



