28 PRESIDENTIAL ADDRESS SECTION IJ. 



having a density (of y.S to 8), approximately equal to that of 

 metallic iron. 



The evidence from earthquakes, therefore, though not 

 altogether conclusive as to the structure of the internal portion 

 of the eartii, is certainly consistent with the conclusion arrived 

 at from the study of radio-activity, of a relatively thin superficial 

 crust, differing essentially in composition from the interior, and 

 probably varying in thickness in different parts of the globe. 



It will be interesting now to inquire how far the facts 

 observed concerning meteorites bear upon the structure and 

 composition of the earth and the idea of a radio-active crust. 

 First of all a few words as to their nature and origin. 



Meteorites are peculiar solid bodies which are continually 

 entering the earth's atmos|)here fnmi outer space, and occa- 

 sionally reach its surface. There are two principal kinds, 

 metallic and stony, and between these there is every gradation, 

 and altogether four fairly distinct classes can be recognised 

 according: to the relative amount of metallic and stonv matter 

 present. These are : — 



T. Holosiderites, consisting almost entirely nf a coarsely 

 crystalline alloy of nickel and iron. 



2. LitJiosiderites, consisting of a nickel-iron matrix, enclos- 



ing granules of basic silicates, such as olivine and 

 l)ronzite. 



3. When the nickel-iron occurs in grains embedded in a 



matrix of the silicate minerals, they arc called Sidcro- 



lifes. 



4. The Sto)ix Meteorites, which are divided into Chondrites 



and AcJiondrites. according t«« the j^resence or absence of 

 i:)eculiar more or less rounded grains of Olixine or 

 Pyroxene, known as (lioiidri or CJioiidndes. 



In addition to the above, there is a remarkable group of 

 bodies, which are supposed 1)y Suess and other authorities to be 

 of extra-terrestrial origin, and therefore, are regarded 1)y them 

 as meteorites. These are called Tektites. and consist of peculiar 

 button-.shaped masses of glass, and have been found in Bohemia 

 and Australia. If these are to 1)e included among the meteorites, 

 they are exceptional, as they contain 80 i)er cent, of silica, 

 whereas the stony meteorites do not contain more than 40 per 

 cent. Others assign a terrestrial volcanic origin to these bodies, 

 but the possibility of their belonging to the meteorites should 

 be borne in mind. 



From the fragmentary nature of most meteorites, it is 

 evident that thev are merely fragments of larger bodies. In 

 some of their characters thev resemble terrestrial rocks, while 

 in others they show striking dift'erences from them, — thus many 

 of the iron meteorites closely resemble the native iron occurring 

 in the Greenland basalts. 



On the whole, nickel-iron is apparently more abundant in 

 meteorites than stony material, the metallic meteorites are usually 



