DIATOMS FROM SOUTH AMERICA. 15 



one-thousandth part of an inch square can be carried three 

 or four hundred miles, and that a narrow strip of vegetable 

 substance something more than half an inch long, and the 

 twelfth part of an inch wide, clearly belonging to some 



Fig. 6. VEGETABLE DEBRIS IN SNOW. (Magnified 500 times.) 



tropical tree, has been carried more than 1,200 miles from 

 any coast where it could have grown, we wonder less at the 

 long journeys taken by these minute one-celled plants, 

 41,000,000,000 of which occupy only one cubic inch of 

 space, and weigh but 220 grains. 



But we have still to speak of that which may more 

 strictly be called the floating matter of the air, that, namely, 



