DEFINITIONS AND LAWS OF GEOLOGY. 9 



§ 11. In high northern and southern latitudes glaciers descend into the sea, 

 where fragments are broken off, which are called "icebergs." Icebergs bear all 

 the earth and rocks they did when constituting part of a glacier, and they soon fall 

 in with ocean currrents, and are drifted great distances before they are dissolved, 

 and let the ' ' moraines " fall to the bottom of the sea. In this manner the sub- 

 marine surface is strewn with foreign mud, sand, gravel, bowlders, and fragments of 

 rock. Coast-ice acts in the same manner when blown out into the sea by off-shore 

 winds. The ice sinks into the ocean eight times as deep as it projects above the 

 surface, and when in shallow water it impinges upon the submarine bottom, the force 

 of the current or the winds may cause it to] polish or groove the rocks, if fragments 

 intervene, in the same manner that glaciers will polish or groove their valleys. Ice- 

 bergs drift from Baffin's Bay to the latitude of the Azores, from Greenland to the 

 mouth of the Gulf of St. Lawrence, from the antarctic regions to the Cape of Good 

 Hope, and also to Chili, in South America. Darwin saw one in the southern seas 

 bearing a rock visible twelve feet above the surface, 1,400 miles distant from any 

 known land. Icebergs have a transporting power more than a thousand-fold greater 

 than glaciers, and an eroding power but little inferior, and yet the action of icebergs 

 is inconspicuous now, and has been, so far as we know, in all the ages gone by. 



§ 12. A large part of the rain sinks into the ground, takes up mineral matter 

 in chemical solution, flows out in springs, and transports its load to the ocean. In 

 this manner many caves and caverns are excavated. The waves produced by 

 storms and tides beat down the shores of large bodies of water, and deposit the ma- 

 terials at other places. The ocean currents have a drifting and denuding action 

 where the water is shallow. The wear and tear of the earth by the action of water 

 never ceases, and the more we contemplate the subject, the better able we are to 

 realize the magnitude of the never-ending destruction. 



§ 13. The violence of earthquakes, and the fires of the volcanoes, the elevations 

 and depressions of land with respect to the sea, seem to have operated within the 

 historical period on as grand a scale as we are warranted in believing they did in 

 past geological ages. Earthquakes and volcanic fires are intimately connected, and 

 neither penetrate the earth to any great depth. Earthquakes have been felt upon 

 the surface of the earth when miners, at a depth of 1,000 feet or more, have not 

 experienced the sensation. The transmission of the vibration is more distinct, and 

 phenomena more apparent where the strata are hard rocks than where they consist 

 of sand and gravel, or softer material. All volcanoes are near large bodies of 

 water, and observation has shown that water gains access to the volcanic foci, and 

 that steam is a powerful agent in all eruptions. The pressure or force of gravity of 

 the layers of the surface of the earth develops the latent heat, so there is an increase 

 of temperature at the rate of about one degree for every sixty feet penetrated for 

 the first 2,000 or 3,000 feet. The deeper borings have not shown the regular con- 

 tinuing increase of the heat, nor is the increase uniform through different kinds of 

 rock, or at different places. The better opinion seems to be that neither this increase 

 of heat, nor the volcanic fires afford any evidence of the internal fluidity of the 

 earth, but, on the contrary, the earth is probably solid, with exception of local 

 caverns near the surface, and local masses of melted matter resulting from chemical 

 causes which are in operation at no great depth. 



§ 14. All are more or less familiar with the story of the buried cities of Hercu- 



