GEOGRAPHY AND ASTRONOMY. 35 



tary rocks uplifted by them, and by the directions which 

 they follow over the long crevices produced by the action. 

 of the forces which have elevated in ridges portions of 

 the crust of the globe. Relations of superposition of 

 trachyte and of syenitic porphyry, of diorite and of ser- 

 pentine, which remain doubtful if studied in the aurife- 

 rous soils of Hungary, in the platinum district of the 

 Oural, or on the South "Western slope of the Siberian 

 Altai, are clearly made out by the aid of observations on 

 the high table-lands of Mexico and Antioquia, and in 

 the unhealthy ravines of the Choco. The most important 

 of the materials which in modern times have afforded a solid 

 basis for physical geography have not been accumulated by 

 chance. In conformity with its characteristic tendencies, 

 our age has recognised, that facts obtained by observations 

 in different regions of the earth, can only be expected to 

 prove fruitful in results, when the traveller is previously 

 acquainted with the state and wants of the science which he 

 seeks to advance, and when his researches are conducted 

 under the guidance of sound ideas, and some insight into 

 the character and connection of natural phenomena. 



By means of the happy, though often too easily satisfied ten- 

 dency towards general conceptions, a tendency dangerous only 

 in its abuse, a considerable portion of the results of natural 

 knowledge may become the common property of all educated 

 persons, producing a sound information very different both 

 in substance and in form from those superficial compilations, 

 which contained the sum of what, up to the close of the 

 last century, was complacently designated by the unsuitable 

 term of popular scientific knowledge. I take pleasure in 

 persuading myself that it is possible for scientific subjects to 



