THE ARGUMENT FROM ANALOGY 367 



ANALOGIES 



The argument from analogy is similar in its import. It is clear from 

 the phenomena of sills and laccoliths that enormous masses of the crust 

 have been moved by intrusions. Laccoliths a mile or two in diameter 

 are common, and their formation required force enough not only to lift 

 "the overlying beds but to bend them at considerable angles. Sills are of 

 much greater extent and are measured in tens of miles, and they per- 

 formed a correspondingly greater amount of work. It may be objected 

 that these are local and not of the same order as the folded mountain 

 ranges. No such objection can be raised, however, to the batholithic 

 intrusions in the southern Appalachians. The longest batholith now 

 known in the Appalachians is 350 miles long, with a width of 30 or 40, 

 which clearly puts the batholiths in the same order of magnitude as the 

 folding which runs parallel with them. These batholiths are composed 

 in the main of biotite granite and have a strong likeness from one end 

 of the Appalachians to the other. The probability is strong, therefore, 

 that some sort of connection existed between the various masses and 

 that the parts now visible are only the tops of far greater masses. Many 

 of them have dome-shaped tops and have pushed up the inclosing gneisses 

 from below. 



Evidence of a similar nature in regard to magnitude is furnished by 

 the enormous outpourings of lava on the surface. Those of India are 

 considered to show a mass of lava of 200,000 cubic miles. A similar 

 order of volume is shown in the Columbia Eiver lava flows with its 

 50,000 cubic miles. Local evidence on this point is furnished by the 

 great system of Triassic dikes and flows of the Atlantic seaboard. These 

 extend in a zone from Georgia to New Brunswick, some 1,200 miles in 

 length, and the width of the belt is from 20 to 100 miles. This implies 

 a large body or connected series of bodies of magma and a force which 

 actuated them through an area nearly as extensive as the Appalachians. 

 Single dikes of this system with a length of 10 miles are numerous, and 

 they have a maximum of 20 miles. 



The phenomena of intrusion shown along the Atlantic coast in New 

 England and Canada are very instructive in regard to the quantitative 

 element in batholithic intrusion. In that region the rocks were scraped 

 clean by continental glaciers and are now kept bare by the waves of the 

 sea, so that they furnish a remarkable series of exposures. The amount 

 of intrusive matter there exhibited is enormous and locally far exceeds 

 in volume the rocks which suffered the intrusion. An estimate of 50 

 per cent of bulk of intrusive matter is reasonable for large areas. 



