Sketch of the Topography and Geology of Lake Ontario. 263 



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and a second equal to 17 fathoms; whence — pr- =1"'2, which 



added to 5" gives about 6", only half the error, or discrepancy 

 observed. Again, The length of a degree at 52° 50' 30" is 

 stated at 60766 fathoms ; and taking this to be the length of a 

 degree employed, we would have, as before, 15", without any 

 attention to the deflection at Arbury, so far as regards the la- 

 titude. Hence it appears probable, that without constantly 

 checking the latitudes by direct observation, an error equal in 

 amount to what we have here found, may easily be com- 

 mitted. It is intended to be distinctly understood, however, 

 that it is not affirmed by me that this is really the cause of 

 the discrepancy I have noticed ; but only as a probable ex- 

 planation of it, which time alone will enable us to clear up. 

 The longitude being still more difficult to verify, any discus- 

 sion of it must for the present be deferred. 



I am, gentlemen, yours, &c. 

 Edinburgh, Dec. 16, 1828. WiLLlAM GalbraITH. 



P.S. If the same rate of error of longitude be allowed on 

 that of Edinburgh, which Dr. Tiarks has found to belong to 

 those of places on the south coast of England, from his chrono- 

 metrical operations at Dover and Falmouth, — it ought to be in- 

 creased by about two seconds ; the longitude would be 12"" 42^ 

 west ; and even this I have reason to believe is nearly a second 

 or two less than the truth. W. G. 



N' 



XL I. A Sketch oftheTopography andGeology of Lake Ontario. 

 By J. J. BiasBY, M.D. F.L. and G.S., For. Mem. Amer. 

 Phil. Soc. Sfc. 



[Continued from page 87.] 



"EARLY the whole of this lake may be safely considered as 

 based upon secondary rocks. Its east end, as far as Hallo- 

 well and Sacket's Harbour, on the north and south shores re- 

 spectively, rests upon strata usually placed in the transition 

 formation ; while its extreme north-east angle is skirted by the 

 j)rimitive ranges of the north, which, stretching from Lakes 

 Huron and Nipissing and from the River Ottawa, cross the 

 outlet of Ontario (the Lake of the Thousand Islands) southerly 

 in a band sixty miles broad, and overspread certain parts of 

 the north of the State of New York. 



Of these three classes of rocks I shall describe the primi- 

 tive first, as far as I am acquainted with it ; but confining my- 

 self to its southern and western borders. Other parts of it are 

 sketched elsewhere. 



All 



