PRESIDENTIAL ADDRESS SECTION 1!. 29 



the larger of the two kinds, and occasionally have fallen in large 

 masses weighing several tons. The stony meteorites are smaller, 

 and probably fall more frequently, but are naturallv more likeh' 

 to be missed than the others, and more likely to become decom- 

 posed and disintegrated after their fall — and so lost. The 

 meteorites actually known probably, therefore, do not give us 

 an accurate idea of their average composition. If this were 

 known it would doubtless be found to show a somewhat lower 

 proportion of iron than has commonly been supposed to be 

 present, and, at the same time, it would give us the average 

 composition of the body or bodies from which the meteorites 

 have been derived. 



Various theories have been advanced as to their origin, 

 but that which best fits with their characteristic features is that 

 they owe their origin to the disruption and fragmentation of 

 some small atmosphereless tody or bodies in space, resembling 

 the satellites or asteroids. They now constitute fragmentary 

 masses, which travel in space in an erratic manner, and with a 

 high velocity which has been estimated in some cases to be as 

 much as 40 or 50 miles per second. 



The meteors which exist as swarms and appear as showers 

 of so-called " shooting stars " at definite times, such as the 

 August and November meteors, are apparently of a dift'erent 

 class, as they follow definite orbits about the Sun, and a])pear 

 to be closely connected in some way with comets, and do not 

 usually reach the Earth. It is f|uite probable, however, that in 

 composition they closely resemble the iron and stony meteorites. 



A brecciated structure is very common in many meteorites, 

 and occasionally these contain fractured chondri among the 

 included fragments. As chondri are structures known only in 

 meteorites, this implies that fracturing or brecciation and re- 

 cementation took place in the parent body ; larger chondri also 

 sometimes enclose fragments of smaller ones. Slickensided 

 surfaces and veins are also sometimes present in meteorites, 

 which implies movements and fracturing in the parent body. 



The very coarse crystallisation of the nickel-iron meteorites 

 indicate that their substance cooled slowly under a high tem- 

 perature and pressure as might be expected to be present in 

 the inner portion of the body from which they were derived. 

 Also the crystal form of the nickel-iron is usually octahedral. 

 which indicates that the metal must have been heated to a tem- 

 ]>erature of, at least, 860° C. before cooling. The occurrence of 

 diamonds, which has been i)roved in two cases, also indicates a 

 high pressure. 



Many of the stony meteorites contain a considerable pro- 

 portion of glassy matter ; this implies rapid cooling, such as 

 would take place at the surface of the parent body, and is 

 analogous to the formation of glass from rapid cooling in 

 terrestrial lavas. 



