442 CHARLES R. KEYES 



Mountains; then comes the Tuertos group; next, the Ortiz, Placer, 

 or Gold mountains; and then, at the north, the Cerillos Hills. 



At the present time each of these mountain groups is a huge, 

 many-peaked boss of augitic and hornblendic andesite. Each also 

 rises out of the Cretacic sandstones which are upturned all around. 

 In some places great intrusive sheets extend out from the central mass 

 to a distance of a dozen miles or more; and immense dikes radiate 

 often to twice the distance named. 



All evidence goes to show that each of the mountain groups is a 

 laccoKth. The molten material, forced upward from beneath, 

 instead of reaching sky floated the overlying strata, forming enormous 



Fig. 7. — Structure of the Ortiz laccolith. 



domes, the tops of which were subsequently removed through erosion. 

 A cross-section of the Ortiz group indicates the structure as repre- 

 sented in Fig. 7. 



The Ortiz group displays to best advantage the various phenomena 

 of laccohthic nature. The Carbonic Hmestones have been com- 

 pletely changed by heat into garnet rock, as is well shown at the Lucas 

 mine on the south side of the great dome. Many dikes and several 

 sills have the Ortiz mass as a center. The largest sill extends south- 

 eastward to the Cerro Pelon, a distance of about twelve miles. This 

 sill is more than 200 feet thick. The great intrusive sheet is of special 

 interest in the present connection for reason of its penetrating the 

 Cretacic coal measures. There are several important coal seams 

 in close proximity to the intrusive sheet. Where the sill has come 



