136 GEOLOGY. 



formations accessible, on the various hypotheses which the diagrams 

 represent. The deepest excavations yet made in the earth are but 

 little more than a mile in depth, and while, by reason of upturning 

 and erosion, rocks which once lay at greater depths below the surface 

 have been exposed, the maximum thickness of rocks open to observa- 

 tion is but a few miles. Knowledge of rock formations and structures, 

 based on observation, is therefore limited to some such thickness. 



Fig. 35. — Diagram representing the structure of the earth according to the planetesi- 

 mal hypothesis. The material of zones 1 and 2 is indicated in the diagram. Zone 

 3 of this figure corresponds to zone 2 of Fig. 34, and zone 4 of this figure corre- 

 sponds to zone 3 of Fig. 34. 



1. On the simpler form of the gaseo-molten hypothesis, we might 

 hope to reach the original crust; for, considering the depths to which 

 rocks are exposed, it is not rational to suppose that this crust, the 

 principal source whence sedimentary rocks were derived, is everywhere 

 covered so deeply by the derivative material as to be inaccessible. 

 2. According to the modified form of the gaseo-molten theory, the 

 oldest accessible rock might not be lower than the deeper portion of the 

 zone of mingled extrusive and sedimentary rocks intermediate between 

 the original crust below and the dominantly sedimentary systems above. 



