102 



SIXTH REPORT — 1836. 



whether it be regarded as error of observation or as resulting 

 from local disturbance : bearing in recollection however, in the 

 case of stations very distant from the central position, that the 

 assumption upon which the equations are founded is probably 

 not strictly correct, viz. of parallelism in the isoclinal lines, and 

 of an uniform rate of increase of dip throughout Scotland. 



The following Table exhibits the differences of observation 

 and calculation ; and shews the geological character of the surface 

 rock at each of the stations, taken from Dr. MaccuUoch's Map, 

 which, as far as I have had opportunities of judging, I have 

 found everywhere most remarkably correct. When the ob- 

 served dip is greater than the computed, the sign + is affixed, 

 and — when less. 



Table III. 



We may divide the differences shown in this Table into three 

 classes ; the first, of seven stations, wherein the differences are 

 veiy great, amounting to 14' and upwards ; second, of eight sta- 

 tions, wherein the differences are more moderate, being between 

 14' and 2' j and in the third class we may place the remaining 

 twelve stations, at which the differences do not exceed 2', being 

 an amount which scarcely deserves to be called a difference. 

 Referring now to the geological characters of the stations, we 

 find, 1st, that all those of the first class, or where the differ- 

 ences exceed 14', are stations either of trap rocks or of rocks of a 

 similar nature ; 2nd, that the stations of moderate differences 

 are for the most part characterized also by the presence of 

 igneous rocks either wholly or partially ; and 3rd, that at all 

 the stations at which the differences do not exceed 2', the sur- 

 face rock is sedimentary. It seems a reasonable inference from 

 these facts, that instrumental errors make but a sukiII portion of 



