VOLCANOES. 701 



Such a cause, working day after day about rocky peaks and preci- 

 pices, causing each day some displacement, may end in degradations 

 of geological importance. Besides shifting the positions of masses of 

 rock, it causes expansion and contraction of thin portions of the exte- 

 rior of rocks, and in some kinds leads to a peeling off of thin layers, 

 as observed by Shaler, or to the opening of delicate fractures that 

 give access to air and moisture for chemical work. 



Among the Thimble Islands, off the shores of Stony Creek, Con- 

 necticut, the walls of granite or granitoid gneiss facing the water, in 

 some of the islands, are peeling off in great laminae, a third to a half 

 inch thick, without any apparent decomposition, or even a dimming of 

 the lustre of the feldspar or mica ; and it may be owing to the heat of 

 the day's sun, and the chilling by the waters when the tide is in. Over 

 the rocky surface of countries within the glacial latitudes of the Glacial 

 period, the scratches left by the glacier are generally, when first un- 

 covered, as fresh and sharp-edged as when they were made. But, if 

 the surface be open to the sun's heat and light, and the rains and frosts, 

 for a dozen years, far the larger part of the markings disappear ; and 

 alternate heating and cooling is an important means of this oblitera- 

 tion of the old markings. 



The sun's heat also produces cracks by drying. Mud-cracks (p. 

 84) are an example. Such cracks in rocks are recognized by their 

 being very shallow ; yet, in the deep soil of some prairies, they extend 

 down two or three yards. 



2. In Solids : the Heat from a Subterranean Source. — From Totten's 

 experiments as data, Lyell has calculated that a mass of sandstone, a 

 mile thick, raised in temperature 200° F., would have its upper sur- 

 face elevated ten feet ; and that a portion of the earth's crust, fifty 

 miles thick, raised 600° F. to 800° F., might produce an elevation of 

 1,000 to 1,500 feet. Cooling again, would reverse the result. 



In the cooling of a rock that has been in fusion, the contraction 

 usually produces fractures at right angles to the cooling surfaces. In 

 this way, in connection with a concretionary tendency in the process 

 of solidification, basaltic columns are produced (pp. 87,112). The 

 cooled mass, when in contact with the adjoining rock, is often much 

 fractured in an irregular way, besides having a finer grain than else- 

 where, in consequence of rapid cooling. Basaltic columns are some- 

 times curved, when the cooling surfaces are not parallel. Sandstones 

 and shales, subjected to a heating and drying, from contact with melted 

 rock, are often fractured in columnar forms. 



3. Expansion and Contraction attending Solidification and Fusion.— 

 Experiments on the contraction attending solidification of rock mate- 

 rial have been made by Bischof, St. Claire Deville, Delesse, and 



