20 GEOLOGY. 



it, stretching out half-way to the nearest stars, its tenuity would be 

 such that, if the orbit of Neptune were the hoop of a drag-net, 5,600,- 

 000,000 miles in diameter, and were sweeping through this space at 

 the rate of 12 miles per second — the approximate velocity of the sun 

 with respect to the stars — it would take some 900,000,000,000 years 

 for it to sweep up the scattered matter. This is probably not an unfair 

 illustration of the average tenuity of the supposed dispersion, since the 

 star-grouping about the sun is probably as dense as the average of the 

 whole. This tenuity immeasurably transcends the best vacuum of 

 the most effective air-pump. It would be an extravagant license in the 

 use of terms to call this a "meteoritic plenum/' Misconception here 

 is fundamentally dangerous. The matter in the heavens is extremely 

 small in proportion to the space — almost inconceivably small. This 

 extreme sparseness of distribution means that the potential energy in 

 the stellar system is enormously great in proportion to the amount of 

 matter. The factors then that weigh in the study of meteoritic assem- 

 blage are not simply, or even predominantly, matter and gravitation, 

 but also, and particularly, space and motion. 



The time factor. — With such excessive tenuity of dispersion, even 

 when all known matter is converted into meteorites, and with such 

 potent obstacles to assemblage as are imposed by the high kinetic 

 energy of the meteorites, it seems an imperative conclusion that the 

 growth of a meteoritic assemblage having the mass of the solar system 

 must require a period quite beyond comprehension. This leads on 

 to the question whether a swarm of meteorites could perpetuate itself, 

 as a swarm, through such a prodigious period. Must not the part 

 first assembled pass on through its own evolution, whatever that may 

 be, without awaiting the excessively delayed assemblage of the later 

 portions? If the members of the swarm were in collisional relations, as 

 postulated by Lockyer and Darwin, must not the kinetic energy of the 

 earlier assemblage have been exhausted long before the accession of the 

 later part? In other words, must not the first assemblage become 

 solid at a relatively early stage in the process, and the remainder of the 

 accessions be added individually, as meteorites are now added to the 

 sun and planets? Is it a tenable view that the mere assemblage of a 

 swarm should go on, without attendant evolution, until the mass 

 necessary for a solar system is attained, and then, but not until then, 

 enter upon an evolution into a sun-and-planet system? If the swarm 



