120 W. Cross — Igneous Rocks of Mexico. 



magnetite in usual form, but a black substance occurs in angu 

 lar spaces between other minerals. This seems to be secondary. 

 There is no glass base. 



Composite quartz grains are surrounded by zones of augite 

 as described by Iddings, Diller and others. 



2. Diorite from Sierra Mercado of Monclova. — A quartzose 

 hornblende-diorite. The rock is a mass of small stout plagio- 

 clase crystals and irregular hornblende prisms pressed close 

 together with quartz as the chief cementing substance though 

 there is probably a little orthoclase, and plagioclase also, as 

 interstitial matter. This rock is quite different mineralogically 

 from the cliorite of Sierra Canclella. 



3. Hornblende-jporijhyrite. — The rock has hornblende and 

 plagioclase phenocrysts in a prominent finely granular crypto- 

 crystalline groundmass. It might be a porphyritic equivalent 

 of diorite No. 2. 



4. Vein matter in diorite. — A fine-grained mixture of 

 orthoclase and quartz. Has no microcline or micropegmatite, 

 or plagioclase. Is like vein matter commonly found in granite 

 or diorite masses. 



5. Hornblende-porphyrite. — Hornblende and plagioclase are 

 the main phenocrysts and the groundmass is unevenly granu- 

 lar, orthoclase being the chief substance. There is very little 

 if any quartz. 



7. Augite-diorite. — Sierra Candella. This rock is chiefly 

 made up of a dark green augite in imperfect prisms, plagio- 

 clase in tablets with very strongly marked zonal structure, and 

 orthoclase, the latter surrounding the plagioclase in a regular 

 growth. There is some biotite, occasionally appearing in 

 blades nearly an inch long ; hornblende is intergrown with 

 augite in the outer zone of some crystals. It is never very 

 intricately intergrown with augite. Quartz occurs in a few 

 small grains, and magnetite, titanite, apatite and zircon, are 

 accessory as usual. 



This rock differs in composition from the other diorite in 

 the presence of augite in place of hornblende, and in the 

 abundance of orthoclase, 



These diorites are deep-seated eruptives similar in composi- 

 tion to several Colorado occurrences which were erupted m 

 some period not long after the Cretaceous, as they cut upper 

 Cretaceous strata, but are affected by orographic movements 

 of early Tertiary time. 



The porphyrites are no doubt equivalent to the diorites, but 

 from smaller masses and perhaps consolidated nearer the sur- 

 face. 



