280 REPORT OF NATIONAL MUSEUM, 1904. 



Like var. A, b, it is rarely cellular in large masses without also containing amyg- 

 daloidal nodules, when it passes to another division. The colors of this variety are 

 usually ash or gray of different hues, or modifications of red, or brown, or purplish 

 blarck. 



The prevalent ideas on the subject of earthquakes at this time may 

 be gleaned from a paper by Dr. Isaac Lea, of Philadelphia, in the 

 American Journal of Science for 1825. Lea rejected the theories put 

 forward in 1749 by a Doctor Stukley to the effect that 

 Earthquakes, 1825. earthquakes were caused by lightning, but regarded 

 them as the effect of volcanic eruptions, though at the 

 same time he could see no necessary connection between the electric 

 discharges of volcanoes and the earthquakes, regarding such as an 

 effect rather than a cause. 



He thought that there were strong reasons for believing a consider- 

 able portion of the interior of the earth to be in a constant state of 

 incandescence and that there were beneath the surface great cavities 

 or channels of connection between one volcano and another. "We 

 must admit," he wrote, "of deep-seated channels of connection 

 stretching from one portion of the globe to the others, through which 

 the explosive gases pass with an instantaneous motion, accompanied 

 by a rumbling or terrible noise peculiar to earthquakes/ 1 In this, it 

 will be noted, he followed to a large extent the teachings of Humboldt 

 and Newton. These channels, he thought, had connection with the 

 sea, basing his opinion upon the fact that volcanoes throw up salt 

 water and fish; moreover, that most volcanoes are situated near the 

 sea. He regarded these branching channels as species of horizontal 

 volcanoes, their roofs and walls furnishing material which, aided by 

 oxygen supplied by the influx of the sea, would yield the necessary 

 amount of combustible matter to keep up the earth's internal fire. 

 To the possible objection that rocks in themselves would not burn, he 

 answered that, when such are decomposed, "their metallic bases, cal- 

 cium, silicium, aluminum, magnesium, etc., are highly incandescent." 



Four years later (1829) Maclure, in discussing the theories of Poulett 



Scrope, took exception to the views of Lea, though he failed to offer 



anything satisfactory in return. He instanced the earthquake of 



1811-12 in the Mississippi Valley, in which only the 



Maclure's Criticisms . i i -f tii i • 



of scrope and Lea, alluvial formations had been disturbed, and questioned 



1829 



if such could not have been caused by the evolution of 

 elastic gases arising from the fermentation of large masses of vegetable 

 matter accumulated in the beds. If such were the case, however, he 

 recognized the possibility of an increase in the frequency of earth- 

 quakes as fermentation went on. Maclure evidently did not place a 

 great amount of reliance on his own theories, for he states, in referring 

 to some of the ideas advanced by Scrope: a "All these speculations are 



a Scrope' a Theory of Volcanoes appeared in 1825. 



