W .1 MCGEE — A FOSSIL EARTHQUAKE. 413 



appear and finally culminate in that part of the scarp overlooking the lake. The 

 narrowest divides are longitudinally cleft by trenches yards in width and often 

 several feet in depth; the steeper outlying. cusps are divided by similar trenches, 

 and frequently the salients are cleft by such trenches cutting across their steeper 

 slopes or radiating in three <>r four lines from the apex. Furthermore, along the 

 face of the scarp opposite the lake, ancient landslips, with their characteristic 

 deformation of the surface, are found in numbers, and over the landslips and along 

 the sides of the trenches on the summit trees are frequently thrown out of the 

 perpendicular. These features suggest a sudden and violent movement by which 

 the highly unstable topographic forms of the upland scarp were in part broken 

 down arid thrown into more stable positions. On examining the inclined trees it 

 is usually found that the great holes two or more centuries old' are inclined from 

 root to top, though the younger trees of seventy or seventy-five years usually stand 

 upright, and that the trunks of a century to a century and a half in age are com- 

 monly inclined near the ground, but are vertical above. Thus the forest trees flank- 

 ing the fissures and clothing the scarp give a trustworthy and fairly accurate date for 

 the production of the minor topographic features — a date determined by much 

 counting of annual rings to lie between seventy-five and eighty-five or ninety 

 years ago. 



While in general the flood-plain of the Mississippi is essentially alike in compo- 

 sition from Baton Rouge to Cairo, a minor distinction is found over the Lake 

 county dome and in its vicinity: The flat-lying alluvium is sometimes interrupted 

 by irregular ridges of gravel and coarse sand, sometimes by double ridges with 

 irregular trenches between; and now and then elongated mounds of similar gravel 

 and coarse sand occur, either isolated or in lines. When in cultivated lands, the 

 ridges and mounds are reduced but give character to the soil throughout entire 

 fields ; but when in woodland, they affect the forest to the extent that the larger 

 trees along their flanks are frequently thrown out of the perpendicular as are the 

 trunks of the upland, while only young trees about seventy-five years old and less 

 grow in the trenches. 



Now it is conceivable that areas may be lifted like the Lake county dome or 

 depressed like Reelfoot lake by gentle diastrophic action ; it is conceivable even 

 that a group of contemporaneous landslips and fissures in a hilly scarp might he 

 developed by a variety of causes or movements coinciding fortuitously; but the 

 gravel ridges and mounds of the flood-plain are homologous with the craterlets, 

 sand spouts and fissures produced by earthquakes, and they are unlike phenomena 

 produced by any other known cause. Moreover the landslips and trenches of the 

 scarp are more perfectly and simply explained as the product of an earthquake 

 than in any other way. Again, the depression of the land beneath Reelfoot lake 

 is analogous to surface movements known to be produced by earthquakes; and it 

 might be shown through the application of the principle of isostasy that the uplift 

 in the center of the Mississippi lowland with the depression on both flanks toward 

 the more heavily loaded uplands, is mire readily explicable as an earthquake 

 product than in any other way. Accordingly the assemblage of phenomena may 

 he explained on the hypothesis of an earthquake of great severity, and cannot 

 well be explained on any other hypothesis. Thus the peculiar features of Lake 

 county, Tennessee, and contiguous territory, may justly be regarded as a physical 

 record of a great earthquake, the date of which is lixed by attendant phenomena 

 at from seventy-five to eighty-five years ago. 



