LUMINOUS SHOWERS. 151 



crystalline, double refracting silicate, drenched through with 

 an ill-smelling organic substance. The dust was found in 

 large quantities at the bottom of innumerable small holes 

 in the surface of the inland ice. This dust could scarcely 

 be of volcanic origin, because by its crystalline structure it 

 differs completely from the glass dust that is commonly 

 thrown out of volcanoes, and is often carried by the wind 

 to very remote regions; as also from the dust which, in 

 March, 1875, fell at many places in the middle of Scandi- 

 navia, and which was proved to have been thrown out by 

 volcanoes in Iceland." Professor Nor den skj old's estimate of 

 the quantity of dust shows that it has been in past ages a 

 not unimportant factor, perhaps, in its addition to the crust. 

 He says, "I estimate the quantity of the dust that was 

 found on the ice north of Spitsbergen, at from .01 to 1 milli- 

 gram per square metre ; and probably the whole fall of dust 

 for the year, far exceeded the latter figure. But a milligram 

 on every square metre of the surface of the earth amounts 

 for the globe to five hundred million kilograms (say half a 

 million tons). Such a mass, collected year by year during 

 the geological ages, of a duration probably incomprehensible 

 by us, becomes a consideration too important to be neglected, 

 when the fundamental facts of the geological history of our 

 planet are enumerated. A continuation of these investi- 

 gations will perhaps show that our globe has increased 

 gradually from a small beginning to the dimensions it now 

 possesses ; that a considerable quantity of the constituents 

 of our sedimentary strata, especially of those that have been 

 deposited in the open sea far from land, are of cosmic origin ; 

 and will throw an unexpected light on the origin of the fire- 

 hearths of the volcanoes, and afford a simple explanation 



