64 



dykes in Anglesea, Newcastle, and Durham ; they afford abun- 

 dant evidence of their aqueous nature and growth from a semi- 

 fluid base, and the effects at the points of contact are such as 

 would naturally take place by dissimilar substances in a moist 

 state, subject to a slow chemical action. 



CHAPTER X. 



POLARITY OF EARTHQUAKES. 



If any persons should be more in favour of the igneous theory 

 than others, it might be supposed that they would be those 

 accustomed to earthquakes and volcanic actions ; however, nine 

 years 5 residence on the Andes has produced on the writer, as 

 will be observed, the very contrary impression. As we have 

 already stated, one striking peculiarity in the American earth- 

 quakes is their meridional action from south to north. The 

 shock of the earthquake of Chili in 1822 was felt simultaneously 

 throughout a space of 1200 miles, from south to north; the la- 

 teral oscillations being confined to narrow bands. The following 

 are a few registered by the author : — 



1834. Jan. 20. — Violent shock from Chili to St. Martha in a 



more or less meridional direction; caused great damage 

 in the line of its maximum force. Its meridional dis- 

 turbances were felt over a space of 1500 miles of linear 

 measurement; but the transverse oscillations scarcely 

 extended over 200 miles on land. 



1835. Feb. 20. — Ditto, a great number, and continued more or 



less daily until the month of March. 



1836. Jan. 4. — Slight shock from the south. 

 Jan. 8.— Ditto. 



On the central Cordillera, from four to five shocks al- 

 most daily, from the 1st to the 15th. 



1837. Nov. 10. — A shock from the S.S.W., accompanied by sub- 



terranean thunder, after heavy rains. 

 Nov. 27.— Ditto, after a few days intense heat. 

 ] 838. June 19. — Two violent shocks; no oscillations; the ground 

 appeared to heave up. 



