HYPOGEIC WORK. 345 



VI. EARTH-SHAPING, MOUNTAIN-MAKING, AND THE ATTENDANT 

 PHENOMENA : HYPOGEIC WORK. 



The preceding chapters on the origin of geological phenomena treat of 

 the agencies by which rocks were made, denuded, crystallized, and filled 

 with veins and ores. The subject of the present chapter is the nature and 

 origin of the changes through which the earth has received its form and 

 features, hypogeicworkj of which the orogenic part is the most noticeable. 

 It does not comprise the work of waters in giving mountain-like shapes 

 to plateaus, and thus producing mountains of circumdenudation, or in making, 

 by accumulation, hills of detritus ; nor the work of heat in building up vol- 

 canic cones, the earth's mountains of igneous accum^idation, or in making 

 laccolithic domes or masses (laccoliths) — mountains of subterranean igneous 

 accumulation; for these operations have already been considered; but work 

 that is consequent, whatever its source, on crustal and interior movements 

 in the earth, as expressed in the term Hypogeic, from the Greek inro, beneath, 

 and yrj, the earth. The attendant phenomena comprise fractures of the earth's 

 crust and supercrust, dislocations, flexures, crystallization and alteration of 

 rocks, rock-melting, and other effects. 



The facts and explanations here presented are supplemented in the fol- 

 lowing pages on Historical Geology, and the chapter will be best under- 

 stood if those pages have already been made familiar. 



Actuality of Changes of Level. 



All geological history testifies against the stability of the rocky crust of 

 the globe ; and if the earth, as is believed, has cooled from fusion, abundant 

 reason for this unstableness exists ; for the effects of the earth's slowly pro- 

 gressing refrigeration reach backward indefinitely, and downward beneath 

 all other agencies of change. 



But the evidence of instability, although the fact is so obvious, is beset 

 with doubts as to amount and position, because of possible and actual varia- 

 tions in the base from which measurements are naturally made. This base 

 is the water-line about the land. Hence, we have to consider the sources of 

 variation in sea level. 



1. Changes in the level of the sea-bottom. — When water-made strata full 

 of marine fossils are found at a height of 1000 feet above the sea, the 

 evidence of a rise of at least 1000 feet appears to be plain. Yet, a lowering 

 of the sea-bottom might produce the same result; and it may, therefore, be 

 a question whether in such a case part, or all, of the ap)parent upward change 

 has not been so produced. 



So, also, by a reverse movement in the sea-bottom, an apparent subsidence 

 might result. Here there is actual change of level, but it may be thousands 

 of miles away from the land along which the change is made visible. Change 

 so caused will affect all seacoasts alike; and in this fact a criterion exists for 

 judging of its reality. 



