86 



LYELL'S ELEMENTS OF GEOLOGY. 



Erratic Blocks drifted by Ice. 



Erratic blocks. — The great size of the boulders sometimes 

 found associated with ancient alluviums, in places between which 

 and the parent rock deep valleys, and even seas, now intervene, 

 has been thought by some to offer insurmountable objections to 

 any theory which does not introduce causes of great violence to 

 account for their removal. These blocks, called erratic, are 

 some of them a few feet, others several yards, in diameter. 

 They are strewed by myriads over the sandy countries of the 

 north of Germany, and parts of Sweden, Denmark, Finland, 

 and Russia. Some of them at least, must have been carried into 

 their present position since the commencement of a very modern 

 geological period, for they rest, near Stockholm, and elsewhere, 

 on layers of sand and marl containing shells of the species now 

 inhabiting the Baltic. 



Although these erratics are far more numerous in northern 

 countries, some are met with as far south as the Swiss Jura, hav- 

 ing evidently been carried thither from the Alps, a chain which 

 is now separated from the Jura by one of the broadest and deepest 

 valleys in the world. 



Now it is inconceivable how any velocity of water could con- 

 vey some of these huge masses, over seas and valleys, to the 

 places where they are now found; but there is no real difficulty 

 in supposing them to liave been carried by ice, when the lands 

 over which they lie scattered were submerged beneath the sea. 



As the reader may perhaps be incredulous respecting the ade- 

 quacy of the cause here alluded to, I shall enumerate many facts 

 recently brought to light, which incontestably prove how impor- 

 tant a part ice plays in the transfer of alluvium from place to 

 place, and especially of that containing large masses of rock. I 

 must confine myself, however, to a brief description of a few 

 examples, as it is not the object of the present work to treat at 

 large of the changes illustrative of geological phenomena, now 

 known to be in progress on the earth. • 



First, in regard to the distribution of erratics ; they occur, 

 both in the northern and southern hemispheres, between the for- 

 tieth parallels of latitude and the poles, but are not met with in 

 the intermediate equatorial and warmer regions.* This fact at 

 once raises a presumption that the greater warmth of parts of 

 Asia, Africa, and America, nearer the line, has proved unfavoura- 

 ble to the transport of such blocks. On the other hand, they 

 abound in the colder regions of North America, from Canada 

 northwards, as well as in northern Europe ; and when we travel 

 southwards, and cross the Line in South America, we fall in 



See Darwin, p. 289, on fsome supposed exceptions to this general rule. 



