rocks. 349 



pact, that they are not visible. Much of the crystalline rock 

 called basalt is thus compact. 



Positions, or modes of occurrence of Rocks. A great part 

 of the rocks of the earth's surface constitute extensive beds 

 or layers, lying one above the other, and varying in thick- 

 ness from a fraction of an inch to many scores of yards. 

 There are compact limestones, beds of sandstone, and shales 

 or clay slates, in many and very various alternations. In 

 some regions, certain of these rocks, or certain parts of the 

 series, may extend over large areas or underlie a whole 

 country, while others are wholly wanting or present only iw 

 thin beds. The irregularities in their geographical ar- 

 rangement and in the order of superposition are very nume- 

 rous, and it is one object of geology to discover order amid 

 the apparent want of system. Thus in Pennsylvania, over a 

 considerable part of the state, there are sandstones, shales, 

 and limestones, connected with beds of coal. In New York 

 there are other sandstones, shales, and limestones, without 

 coal ; and the geologist ascertains at once by his investiga- 

 tions, (as was observed in the remarks on coal,) that no coal 

 can be expected to be found in New York. These rocks 

 contain each its own peculiar organic remains, and these 

 are one source of the confident decision of the geologist. 

 The stratified rocks bear evidence in every part — in their reg- 

 ular layers, their worn sand or pebbles, and their fossils, — 

 that they are the result of gradual accumulations beneath wa- 

 ter, marine or fresh, or on the shores of seas, lakes or 

 rivers. 



Besides the stratified rocks alluded to, there are others 

 which, like the ejections from a volcano, or an igneous vent, 

 form beds, or break through other strata and filHissures often 

 many miles in length. The rock filling such fissures, is 

 called a dike. Such are the trap dikes of New England 

 and elsewhere ; they are fissures filled by trap. Porphyry 

 dikes, and many of the veins in rocks, are of the same kind. 

 Similar rocks may also occur as extensive layers ; for 

 the lavas of a single volcanic eruption are sometimes con- 

 tinuous for 40 miles. They may appear underlying a wide 

 region of country, like granite. 



The stratified rocks, or such as consist of material in reg- 

 ular layers, are of two kinds. The worn grains of which 

 they are made are sometimes distinct, and the remains of 

 shells farther indicate that they are the result of gradual accu- 



