CONSTRUCTION OF THE EARTH. 149 



the dikes. It is a banded arrangement that is, the ma- 

 terials are in bands parallel with the walls of the vein. 

 The bands may be few in number or numerous. If there 

 be metallic ore in a banded vein the rocky material is 

 called the vein-stone or gangue. 



Dikes are due to the injection upward of melted ma- 

 terial into openings or fissures in the rocks ; but the man- 

 ner in which veins have been produced is by no means as 

 yet fully understood, and there have been much discus- 

 sion and speculation about it. 



The distinctions between dikes and veins are generally 

 recognized among geologists as I have pointed them out, 

 but sometimes there is a little confusion in the use of 

 these terms. The term lode is often given to veins, but 

 seldom to any except those which are metallic. 



232. Drift This term is applied to fragments of rocks 

 which have been scattered over some portions of the 

 earth's surface by other means than the flowing of water. 

 The fragments vary in size from grains up to those which 

 are many tons in weight. When they are of any consid- 

 erable size they are called boulders. They are supposed 

 to have been scattered by means of icebergs or glaciers, 

 or both. Drift is not found at all in tropical climates. 

 In Xorth America it appears in Canada, New England, 

 the State of Xew York, and in all the states "west of that 

 region in the same latitude. In Europe also it is con- 

 fined to the northern part. From comparison of the 

 drift with the stationary rocks, and from other observa- 

 tions, it is manifest that all the drift came from the north. 

 In the southern hemisphere, however, it came from the 

 opposite direction. In both cases its,movement was from 

 the pole toward the equator. The distance to which 

 boulders have been carried is sometimes very great, even 

 hundreds of miles. We know where they came from by 

 their composition and other circumstances. Sometimes 

 long trains of boulders are traced, as if an iceberg slowly 

 sailed along, dropping continually as it went its rocky 



