IGNEOUS ROCKS FROM THE ORTIZ MOUNTAINS 231 



This laccolith is the largest of four, lying in a north and south 

 line. The northernmost, the Cerrillos Hills, has been described 

 by D. W. Johnson.^ The Ortiz is the next group, while the San 

 Pedro and the South Mountains make up the southernmost groups. 

 It is apparent that these groups are closely related and they probably 

 constitute a single comagmatic region. 



The scope of the present paper is primarily chemical. A brief 

 statement of field relations, texture, and mineralogy will be given for 

 each type analyzed, but a detailed discussion of optical characters 

 will not be undertaken in this preliminary paper. It is probable 

 that all the types here discussed are of post- Cretaceous age. 



Laurvikose — Lassenose I. 4-5.2.4 (Dacite) 



Rocks of this type are gray, porphyritic, and form sheets on all 

 sides of the main mass. The phenocrysts are of pink or white 

 feldspar, the ground mass is phanerocrystalline but iine-grained. 



Microscopically the phenocrysts are found to be soda orthoclase 

 with more or less concentric structure. A few phenocrysts of green 



TABLE II 

 Chemical Composition of Lassenose 



I School of Mines Quarterly, 1903-4. 



