2S6 SCIENTIFIC APPARATUS. 



tures by which the rocks of the earth's crust have been dislocated, 

 and can measure the amount of displacement to the extent, some- 

 times, of many thousand feet. He finds that, as at the present 

 day numerous volcanic orifices exist from which hot vapours, 

 gases, dust, and molten matter are ejected, so in the various 

 epochs of past geological time the earth's surface has been simi- 

 larly dotted over with active volcanoes. He can follow the 

 course of long-buried lava streams, and, by help of the fossils 

 associated with the beds of compacted dust, can tell whether 

 the eruptions took place on land or sea. He everywhere en- 

 counters evidence that the present hills and valleys of the land 

 bear witness in their internal structure to former wholly differ- 

 ent conditions of the surface, and in their external outlines to 

 the slow but powerful influence of the rains, frosts, and other 

 agencies by which even the most solid rocks are crumbled 

 into ruin. 



That the observations which lead to these and other . con- 

 clusions of Geology with regard to the past history of the earth 

 may be accurately made, it is needful that geological maps should 

 be constructed to show the actual or inferred arrangement and 

 distribution of the rocks of a district or a country. On these maps 

 the area respectively occupied by the various rocks must be deline- 

 ated, at the same time the angle and direction of the inclination 

 of the strata, the position, trend, and amount of throw of the dis- 

 locations, with such other details as may be requisite to obtain a 

 clear idea of the geological structure of the ground. The most 

 detailed geological maps yet constructed are probably the large 

 sheets of the Geological Survey of Great Britain and Ireland on 

 the scale of six inches to the British statute-mile, or Toewof 

 nature. Most of the civilised states of the world have organized 

 national geological surveys, primarily to make known their mineral 

 resources, though at the same time usually tending to advance the 

 interests of pure unapplied geology. 



In geological surveying, the first essential is the possession of a 



