170 WHAT IS LIFE ? 



them. Most probably the earth is a hollow spheroidal 

 mass. 1 Now inasmuch as these igneous rocks rise to 

 the surface of the earth, it is most likely that often the 

 whole thickness of the crust of the earth may be igneous 

 rocks ; if so, how enormous must be the thickness ! 



If the earth were once wholly incandescent, then all 

 sedimentary rocks must have been formed from igneous 

 rocks rocks which must have been reduced to ex- 

 ceedingly fine particles to form the various clays, &c., 

 which we find in Nature. 



We cannot therefore tell the origin of the earth, or 

 even of the igneous rocks. It is enough for man to 

 know that they are. 



Thus, then, lying over these ancient " igneous " and 

 " metamorphic " rocks, which often consist of granite, 

 are the various formations to which we have just drawn 

 attention. It is estimated by geologists that these 

 ante-primary rocks which have been partly altered into 

 igneous rocks are sometimes 30 ? 000 feet thick. The 

 primary rocks which are deposited on the top of the 

 ante-primary rocks are found 106,000 feet thick, 

 making a total of about 136,000 feet, or nearly twenty- 

 six miles. The secondary rocks which are deposited 

 on the top of the primary rocks may be met with 

 25,000 feet, or nearly five miles thick. The tertiary 

 rocks which are laid upon the secondary rocks are 

 sometimes 27,000 feet, or over five miles thick. While 

 the post-tertiary layers, the last deposits of all, are 

 found 500 feet thick. 2 These are very approximate 

 measurements. There are no hard and fast lines the 



1 For the reasons see "What is Heat ? " p. 104. 



2 "The Story of our Planet." T. G. Bonney, D.Se., LL.D., 

 F.R.S., 1897, p. 365. 



