THE TERRESTRIAL DISTRIBUTION OF RADIUM 31 



subsequent to solidification, or to (c) the effects of high pressure. 

 The second view supports the hypothesis that meteorites 

 are the scattered fragments of a suddenly disrupted and there- 

 fore suddenly chilled cosmic body; the first and last that 

 holosiderites came from the central core of such a parent body, 

 i.e. where cooling would be long delayed, and mechanical 

 pressure would be at its maximum. 



The octohedral form of many of the crystals betrays in part 

 their past thermal history, for it proves that the temperature 

 which conditioned their growth must have exceeded 86o° C. 

 One effect of high pressure would be, of course, to raise this 

 lower limit very considerably. 



Lithosiderites are meteorites which consist of a nickel-iron 

 matrix containing granules of basic silicates such as olivine 

 and bronzite. When the silicate minerals preponderate over 

 the metallic alloy so that the latter occurs in grains embedded 

 in stone, the meteorite is known as a siderolite. The nickel- 

 iron in these two types is also generally of octohedral form, 

 and exhibits the Widmanstatten figures. Equally significant 

 is the presence of tridymite, a crystalline form of silica 

 which is stable between temperature limits of 8oo° C. and 

 1,625° C. 



The stone meteorites proper are divided into chondrites and 

 achondrites according to the presence or absence respectivel} 7 

 of peculiar rounded masses of olivine or pyroxene, which are 

 known as chondri or chondrules. The origin of these puzzling 

 structures as yet is not understood, for they have no known 

 terrestrial analogues. 1 Some of the meteoric stones have cer- 

 tainly crystallised from a molten magma, and may be paralleled 

 with the ultra-basic rocks. Others, however, have a clastic 

 or fragmental structure, and seem to be of the nature of volcanic 

 tuffs and breccias, to which they bear a close resemblance. 

 The common presence of a crypto-crystalline matrix, or a 

 dark basic glass, affords clear evidence of rapid cooling. These 

 features point to the superficial conditions under which some 

 of the stones were originally formed. 



Dr. Prior 2 has recently shown ,that there is a striking 

 similarity both in the chemical and mineralogical compositions of 



1 For an interesting suggestion see Fermor, Rec. Geol. Surv. India, vol. xliii. 



1913, PP- 41-47. 



2 Mi?i. Mag. vol. xvii. p. 33, 1914. 



