70 LITHOLOGICAL GEOLOGY. 



to silica (quartz) by a silicifying process. Silicified trunks of trees, 

 as well as shells, occur of all geological ages. In some cases fossils 

 have been altered to an oxyd of iron, or to the sulphuret of iron 

 (pyrites). 



In many cases the fossils are entirely dissolved out by percolating 

 waters, leaving the rock full of cavities. This happens especially 

 in sandstones, through which waters percolate easily, and not in 

 clays, which preserve well the fossils committed to them; and 

 hence sands, gravel, conglomerates, quartzose sandstones, contain 

 few organic remains. 



3. KINDS OF ROCKS. 



79. General subdivisions. — Eocks are conveniently divided into 

 fragmental and crystalline. 



1. Fragmental. — Eocks that are made up of pebbles, sand, or clay, 

 either deposited as the sediment of moving waters or formed and 

 accumulated through other means : — as ordinary conglomerates, sand- 

 stones, clay-rocks, tufas, and some limestones. The larger part of the 

 rocks here included are made of sedimentary material, and are 

 commonly called sedimentary rocks. They are stratified rocks, — that 

 is, consist of layers spread out one over another. Many of them 

 are fossiliferous rocks, or contain fossils. 



2. Crystalline. — Eocks that have a crystalline instead of a frag- 

 mental character. The grains, when large enough to be visible, 

 are crystalline grains, and not water-worn particles or fragments of 

 other rocks. Examples, granite, mica schist, basalt. 



The crystalline rocks may have been crystallized, — 



a. From fusion, like lava or basalt, when they are called igneous rocks. 



b. From solution, as with some limestones. 



c. Through long-continued heat without fusion. By this last 

 method sedimentary beds have been altered into granite, gneiss, 

 or mica schist, and compact limestone into statuary-marble. 



As, in such cases, a bed originally sedimentary has been meta- 

 morphosed into a crystalline one, rocks of this altered kind are 

 called metamorphic rocks. 



In the following descriptions a separate subdivision is made of 

 the calcareous rocks or limestones, which are mostly sedimentary in 

 original accumulation, but generally lose that appearance as they 

 solidify. 



80. Characteristics of Rocks. — Independently of the characters 

 above mentioned, rocks differ in kinds : — 



