30 Field Columbian Museum — Geology, Vol. i. 



at these intervals. It had previously been seen in 1772 and 1805 and 

 returned to the solar system in due order in 1832. Being in an un- 

 favorable position in 1839 it could not be seen, but at the time of its 

 next appearance in 1846 it was found to have separated into two por- 

 tions, which kept drifting - farther apart during the time in which the 

 comet remained visible. At its next appearance in 1852, the frag- 

 ments were seen to be smaller and still more widely separated. In 

 accordance with its times of revolution the comet should have reap- 

 peared in 1859, 1866, 1872, 1879 and 1885, but though carefully 

 looked for, ,it has never been seen again. 



On November 27, 1872, however, occured a meteoric shower 

 extraordinary for the number and brilliancy of the meteors which 

 flashed through the air. The orbit of these proved to be exactly that 

 of Biela's comet. On the same date in 1885 occurred another remark- 

 able shower of meteors, having the same orbit and radiant point as 

 those of 1872. During this shower an iron meteorite weighing about 

 8 pounds fell at Mazapil in Mexico. The manifest conclusion, there- 

 fore, is that sometime between the years 1852 and 1872, Biela's comet 

 was shattered in pieces and some of these meteors were the resulting 

 fragments. These fragments being small, were mostly burned up 

 in their passage through the upper part of the earth's atmosphere, 

 but had they been larger, numbers of meteorites would probably 

 have fallen to the earth. 



The fact, however, that so few meteorites have fallen to the earth 

 during the star showers has been urged by some authorities as proof 

 that the meteors producing stones are of a different nature from those 

 which we see only as shooting stars. Since however, every grada- 

 tion may be traced from one to the other and astronomically they are 

 all alike, there is little reason, in the view of many authorities, to 

 doubt their similarity. 



Attention has already been called to the fact that though upon 

 the earth's surface iron is rarely found uncombined, there are masses 

 found in the basalt of Greenland which are altogether metallic and 

 which in composition and structure closely resemble the meteoric 

 irons. Though other views as to their origin have been advanced, 

 many facts point to the conclusion that these iron masses have been 

 brought up with the basalt and therefore indicate the existence of 

 metal of this character in the deep interior of the earth. It has long 

 been known that the matter constituting the interior of the earth 

 must be more dense than that of the rocks which form its crust, since 

 the specific gravity of the earth as a whole is 5.5, while that of the 

 rocks of the crust is not more than 2.7. Professor Dana has shown 



