GEOTECTONICS OF ESTANCIA PLAINS 



443 



directly into contact with the coal seam the latter has been entirely 

 destroyed. When separated by a few feet of shale a fine grade of 

 anthracite is produced. This variety of coal is extensively mined at 

 the town of Madrid and elsewhere in the vicinity. Seams still farther 

 removed from the sill are bituminous in character ; while at a distance 

 of IOC to 200 feet the coal seams are lignitic. 



The relations of the sill to the coal seams suggest that the intrusive 

 sheet itself may have followed along a large coal seam, perhaps the 

 most extensive of all, as a horizon along which it was able to move 



Fig. 8. — Plan of large dike cutting coal-field near Cerro Pelon. 



most easily. Similar phenomena have been observed in the Scottish 

 coal fields and in the coal fields of Germany and Hungary. 



The dikes originating from the Ortiz mass often extend a score 6f 

 miles across the plains as huge walls rising 100 to 200 feet above 

 level surface. Some of these dikes show the amount of lateral dis- 

 placement of the faults. One dike in particular has suffered move- 

 ment in a remarkable way, as shown in the groundplan (Fig. 8) . The 

 squares in the cut are one mile in each direction. 



Relations of the structure to the plains surface. — The even surface 

 of the Estancia plains, vast as it is, is not, according to the strict physio- 

 graphic usage of the term, that of a structural, or stratum-plane, 



