XXxVl SEVENTH REPORT—1837. 
several occasions, what has been remarked by other observers, that 
magnetical experiments are liable to be affected by the vicinity of Trap 
rocks, This was particularly noticed by him at Oban and Loch Scavig, 
so as to render his observations at the latter of no utility for his calcu- 
lations. Two of the most familiar examples of this quality of ignigenous 
rocks are afforded by the powerful effect of a column of the Giant’s 
Causeway, as mentioned by Professor Lloyd ; and by the strong polar- 
ity of the basaltic cap of Arthur’s Seat, near Edinburgh, which is ca- 
pable, in more positions than one, of causing complete inversion of poles 
of the pocket compass. These instances show how carefully the vi- 
cinity of considerable masses of Trap rocks should hereafter be avoided, 
in all delicate experiments on magnetic dip and intensity: for the errors 
they occasion may be more considerable than the effect of a ship’s local 
attraction on azimuths, and are far less easily compensated. 
Major Sabine has considered it best to give no other designation, on 
his chart, to the isodynamic lines in Scotland, than what expresses their 
relation to each other, until we have more fully investigated their rela- 
tion to magnetic intensity in England. The differences between the 
deductions, in regard to the Isodynamic lines in Scotland and in Ireland, 
are very considerable, and apparently too great to be due to any dif- 
ference in the lines themselves: but future observations will probably 
disclose the cause. 
In a former volume of our Transactions, appeared a valuable report 
on North American Geology: in that just announced is an excellent 
essay on the Zoology of that portion of the globe, by Dr. Richardson, 
the intrepid friend and companion of Sir John Franklin, in their ha- 
zardous exploratory expeditions to the shores of the Arctic Ocean. 
After some general remarks on the climate of North America, he pre- 
sents us with an extensive Table of Mean Temperatures, calculated for 
periods of six and three months throughout the year, for the hottest and 
the coldest months, and for the months with a mean temperature above 
52° Faht., taken at forty-four different stations, and collected from his 
own and Franklin’s observations, combined with those of Humboldt, 
Ross, Parry, and Scoresby. The results are very important, and show, 
in a striking manner, the very erroneous deductions on the mean tem- 
perature of any place, if investigated by Mayer’s formula, especially in 
very low or very high latitudes. 
The geographical position of Mexico constitutes the point at which 
the Faune of the northern and southern regions meet; and hence it is 
the place in which the general laws regulating the distribution of animals 
can be most satisfactorily studied. There the Wolf of a northern cli- 
