278 JOURNAL, OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 12 



Do 



B = 



where tan 6 



(2^ \ 



cos \~ t — Q J 





4pT 

 There is thus a lag, and a reduction of amplitude in the ratio unity to 



(i + Tr^aVier^p-)'/^ 



If T is of the order of 500 years, and p corresponds to copper, 

 this ratio amounts to 0.27 X10~"^. Thus, if the resulting apparent 

 magnetization perpendicular to the geographic axis is to be at all 

 comparable with the permanent magnetization (or its equivalent^) 

 perpendicular to that axis, the internal conductivity of the Earth must 

 be smaller than that of copper at least in the ratio 0.3 XI 0~Ho unity. 

 Of course, there is nothing remarkable in this since presumably nobody 

 would wish to consider the conductivity of the Earth's interior as 

 comparable with that of copper; but it is nevertheless of interest to 

 observe that the period of the secular variation has something to say 

 as regards an upper limit to the conductivity in question. 



When one pictures the flow of currents left to themselves in a 

 sphere, he is likely, at first sight, to regard resistance as the main in- 

 fluence controlling them. We see, however, that in the case of a large 

 sphere, such currents are more analogous in their behavior to highly 

 frictionless gyroscopes, and the problems of precession, etc., associated 

 with gyroscopic motions naturally invite attention. 



The pertinence of the above to problems concerning magnetic fields 

 in sun-spots is obvious. 



PETROLOGY. — Preliminary note on monticellite alnoite from Isle 



Cadieux, Quebec.'^ N. L. Bowen, Geophysical Laboratory, 



Carnegie Institution of Washington. 



The occurrence of alnoite in the vicinity of Montreal, Canada, has 



been known for some years. The first discovery was made by Adams, - 



and a paper by Harvie^ describes a number of separate intrusives of 



this nature. During the past summer Doctor Harvie called to my 



'' We are not limited to the case of a rotating permanent magnetization as the active 

 agency. Thus we mighc consider a system of e. m. f.'s of unspecified origin, whose effects 

 were to generate currents which would act as the equivalent of the permanent magneti- 

 zation cited. 



' Received June 11, 1921. 



2 F. D. Adams. Amer. Journ. Sci. (3) 43: 269-279. 1892. 



' Robert Harvie. On the origin and relations of the Palaeozoic breccia of the vicinity of 

 Montreal. Trans. Roy. vSoc. Canada. (3) 3: Sect. 4, 249-299. 1909. 



