58 THE PHYSICAL KINSHIP 



stranded here on this island of cold, when their 

 relatives, on the return of warmth at the close oi 

 the glacial period, retreated back to the ice-bound 

 fastnesses around the pole. It is for a similar 

 reason, probably, that the flora of the upper 

 White Mountains resembles that of Labrador. 



7. One of the strongest pieces of evidence 

 bearing on evolution that is furnished by any 

 department of knowledge is that furnished by 

 geology. It is the evidence of the rocks. Geology 

 is, among other things, a history of the earth. 

 This history has been written by the earth itself 

 on laminae of stone. It is from these records that 

 we learn incontestably the order in which the 

 forms of life have made their appearance on the 

 earth. 



Three-fourths of the surface of the earth is sea. 

 Over the surface of the remaining fourth, except- 

 ing in mountainous places, is a layer of soil, vary- 

 ing from a few feet to a few hundred feet in depth. 

 Beneath this coverlet of soil, extending as far 

 as man has penetrated into the earth, is rock. 

 Excepting in regions overflowed by lava poured 

 out from beneath, or along the backbones of 

 continents where the surface rocks have been 

 upheaved into folds and carried away by denuda- 

 tion, the rocks immediately beneath the soil, to 

 a thickness often of thousands of feet, are in the 

 form of layers, or sheets, arranged one above 

 another. These rocks are called sedimentary 

 rocks, as distinguished from the unlaminated 

 roc'cs of the interior. They have been formed 



