EXTRA-TERRESTRIAL CAUSES 



907 



produced the widespread metamorphism and exterminating most 

 of the organisms which had reached a high state of development 

 prior to the opening of Palaeozoic time. Such catastrophic occur- 

 rences are within the range of possibility, and, indeed, the rising 

 school of astro-geologists regards the origin of the earth itself as 

 due to accretion of planetary matter, rather than to the development 

 by condensation from an original nebular condition. If the accre- 



FiG. 251. Prism of wax, deformed under pressure, between two iron plates 

 (B, B) and exerted in the direction shown by the arrows. Two 

 main sets of fissures are formed nearly at right angles to each 

 other (FF, f) leaving a triangular prism between them. RR, 

 plexus of fine fissures at right angles to one another. Scale i :75. 

 (After Daubree from Haug. ) 



tion theory of earth-building, whether from planetesimals or from 

 meteorites, should become established, the recurrence of such an 

 e\ent — the repetition of the celestial bombardment to which the 

 earth owed its origin — is brought within the range of probability, 

 and thus the stupendous problem of the origin of the great differ- 

 ence everywhere oljserved lietween the Cambric and the undoubted 

 pre-Cambric * rocks may be brought a step nearer solution. 



* Some of the undisturbed clastic rocks now referred to the pre-Cambric 

 have not yet been proven to be such. The occurrence of disconformities between 

 Middle Cambric or late Lower Cambric and underlying unaltered sediments is 

 not conclusive evidence of the pre-Cambric age of these sediments, especially if 

 they are of continental origin. 



