AMERICAN GEOLOGY EATONIAN ERA, 1820-1829. 255 



He did not regard the trap of the Conewago Hills as igneous, but 

 that of Stony Hill presented to him an appearance more decisively 

 volcanic. This did not, however, necessarily mean that there may 

 have once existed a crater here. 



These observations of Gibson relative to the mode of decomposition 

 of the trappean rock are of interest when taken in connection with 

 observations of other individuals of about this period in North 

 Carolina. 



It appears that the attention of the Rev. James Hall and Zachariah 

 Lewis had been called to what are now known to be small trappean 

 dikes on the Yadkin River, near Salisbury, which had undergone 

 decomposition into small bowlder-like masses. The fact that these 

 dikes had at one time occupied rifts in older rocks which had largely 

 disappeared through decomposition, leaving them standing in wall- 

 like masses, entirely escaped the observation of the two gentlemen 

 named, and they were consequently described as probably of human 

 workmanship. 



In response to a request of Dr. Samuel L. Mitchill, Mr. John Beck- 

 with made an examination of the occurrences and reported the result 

 in a letter printed in the American Journal of Science for 1822. 

 Beckwith described in considerable detail the occurrence of the dikes 

 and the stages of transition from the solid bowlder-like masses to the 

 supposed interstitial cement. While apparently not realizing com- 

 pletely the fact that their then appearance might be due to decompo- 

 sition, his description is such as to leave no doubt in the mind of the 

 present reader. As to the exact character of the material itself, he 

 was somewhat at a loss, but, on the whole, inclined to believe it to be 

 basaltic, though perhaps of aqueous rather than igneous origin. 



The work of Thomas Nuttall merits attention here only on account 

 of the time of its accomplishment. Nuttall himself was not a geolo- 

 gist, nor can he be considered an American. His principal work was 

 of a botanical and ornithological nature, the paper, 



Nuttall's . . & ' il \ 



observations, Observations on the Geological Structure of the Valley 



of the Mississippi, being his sole contribution to the 

 literature of geology. This was read before the Philadelphia Academy 

 in December, 1820, and in its printed form occupies 38 pages of the 

 journal of the society. 



Nuttall's travels took him along the southern shore of Lake Erie to 

 Detroit; thence by canoe along the coast of Lake Huron to Michili- 

 mackinac; thence southwestward to Green Bay, Wisconsin, and by way 

 of the Fox and Wisconsin (Ouisconsin) rivers to the Mississippi, near 

 Prairie du Chien, and southward to St. Louis. This and other trips 

 up the Missouri and Arkansas rivers gave him ample opportunity for 

 such superficial observations as he was competent to make These 

 observations consisted largely of conjectures as to the geographic 



