F. W. Clarke — Studies in the Mica Group. 131 



It may be objected, however, that the probability of any 

 tornado occurring at all is so small, that the probability that 

 one will occur in any one of the districts not predicted for is 

 entirely overborne by the former consideration. This objection, 

 however, is only plausible. If we should take the occurrence 

 of a tornado on any single day of a year as a criterion, it must 

 be admitted that the probability of such occurrence is very 

 slight, but in the case before us we are not considering the 

 occurrence of a tornado on any one of 365 days but rather on 

 any one of, say, 50 special days when they are very likely to 

 occur. It is easy to see that in this case the probability that a 

 tornado will occur in any one of a large number of districts, on 

 any one of a small number of special days, exceedingly favor- 

 able for its development, is vastly greater than the general 

 haphazard guess that one will occur on any day of the year and 

 especially on an unfavorable day. It seems as though this 

 important principle has been overlooked in the general discus- 

 sions of this question. 



It seems probable that the division of the country into dis- 

 tricts, in each of which predictions are to be made, is hardly wise. 

 The whole subject is still on the border-line of uncertainty and 

 inde'finiteness. Possibly it would be more satisfactory to pre- 

 dict, in a region where at least 25 or 30 destructive storms and 

 tornadoes occur each year, a central point or locus of destruc- 

 tive storms, giving boundaries, more or less definite, to the 

 limit of destruction, and in verifying to give weights to storms 

 occurring at distances of 50, 100, etc., miles from that locus. 

 It is also essential that we pay the closest attention not to the 

 probability of a tornado occurring on any day in general, but 

 rather to the probability of its occurring on any one of a few 

 special days when the general meteorological conditions and 

 our knowledge of the laws of such storms (for example, their 

 occurring in the southeast quadrant of a low area), would lead 

 us to infer that they are extremely likely to occur. 



Dec. 4, 1885. 



Aet. XVI. — Studies in the Mica Group; by F. W. Clarke. 



I. Muscovite from Alexander County, JV. C. 



This mica occurs in overlapping, crystalline plates, implanted 

 edgewise in pockets at the well-known locality for emerald and 

 hiddenite at Stony Point. The plates are several centimeters 

 in diameter, with sharply defined crystalline boundaries; and 

 in the specimens before me they are associated with crystallized 

 dolomite, pyrite, and rutile. All of these minerals are more or 



