6 PROGKESS : US law and cause. 



further, that it has been becoming more heterogeneous in 

 respect of the composition of these strata, the latter of 

 which, being made from the detritus of the older ones, are 

 many of them rendered highly complex by the mixture of 

 materials they contain ; and that this heterogeneity has 

 been vastly increased by the action of the Earth's still 

 molten nucleus upon its envelope, whence have resulted 

 not only a great variety of igneous rocks, but the tilting 

 up of sedimentary strata at all angles, the formation of 

 fiiults and metallic veins, the production of endless disloca- 

 tions and irregularities. Yet again, geologists teach us 

 that the Earth's surface has been growing more varied in 

 elevation — that the most ancient mountain systems are the 

 smallest, and the Andes and Himalayas the most modern ; 

 while in all probability there have been correspondmg 

 changes in the bed of the ocean. As a consequence of 

 these ceaseless differentiations, we now find that no consid- 

 erable portion of the Earth's exposed surface is like any 

 other portion, either in contour, in geologic structure, or 

 in chemical composition ; and that in most parts it changes 

 from mile to mile in all these characteristics. 



Moreover, it must not be forgotten that there has been 

 simultaneously going on a gradual differentiation of climates. 

 As fast as the Earth cooled and its crust solidified, there arose 

 appreciable differences in temperature between those parts 

 of its surface most exposed to the sun and those less exposed. 

 Gradually, as the cooling progressed, these differences be- 

 came more pronounced ; until there finally resulted those 

 marked contrasts between regions of perpetual ice and 

 snow, regions where winter and summer alternately reign 

 for periods varying according to the latitude, and regions 

 where summer follows summer with scarcely an appreciable 

 variation. At the same time the successive elevations and 

 subsidences of different portions of the Earth's crust, tend- 

 ing as they have done to the present irregular distribution 



