90 STEUCTURAL GEOLOGY. 



corals, and other calcareous relics of living species, these relics of the seas 

 have been ground up, as now in coral and shell-growing seas, and made into 

 limestones ; for limestones are for the most part fragmental rocks. 



The metamorphic crystalline rocks, as already stated, are only fragmental 

 rocks metamorphosed or crystallized. The alteration in some mountain- 

 making epochs has changed fragmental formations thousands of feet in 

 thickness over many thousands of square miles in area. The borders of 

 such areas are usually less altered than the interior portions ; and hence 

 in many places the transition may be passed over, in the course of a score 

 or two of miles, from the simply solidiiied strata of the outskirts and the 

 faintly crystalline slates and limestone, to the thoroughly crystalline mica 

 schist, gneiss, and marble ; and sometimes to granite in masses or veins as 

 an extreme effect. 



Chemical deposits, or deposits from solution in fresh or salt waters, have 

 added sparingly to the stratified series, and the outflows of igneous rocks 

 from fissures or volcanic rents have made other large additions. Part of such 

 ejections go to make independent conical mountains ; but the larger part 

 are in successive sheets interstratified or overlying other formations. 



Of these materials, all are of superficial origin excepting the igneous ; 

 these are contributions to the surface from the earth's interior. 



Besides stratified terranes, there are also vertical or obliquely placed 

 sheets of rock cutting across the former. They are the fillings of opened 

 cracks or fissures made across the terranes, and comprise dikes and veins. 

 They have great geological and economical importance because of the gems 

 and ores which veins and dikes have made accessible to man, and because 

 dikes are the inferior portions of great igneous outflows, and reveal some- 

 thing as to the earth's interior. But they are of small extent compared with 

 the stratified terranes, and will be considered under Dynamical Geology. 



Formations. — From the explanations that have been given it is apparent 

 that any group in the series of stratified rocks, whether large or small, may 

 be called a formation, if the parts are related in period or time of origin ; 

 as, for example, the Devonian formation, or those of the Devonian era ; the 

 Chemung formation, or those of the Chemung period under the Devonian ; 

 and so on. The term is also used for a group of rocks of similar constitu- 

 tion; as a calcareous formation, a siliceous formation, etc. The term terrane 

 (from the Latin terra, earth, and the French terrain) has essentially the 

 same signification as formation. Formation is commonly used for stratified 

 terranes. 



Stratified FoRMATioisrs. 



1. Structure and Characteristics. 

 The series of stratified formations over the globe has a maximum thick- 

 ness of about 30 miles. But the existing thickness in any one place is 

 seldom even 10 miles. Since rocks are mostly water-made, and for the 

 larger part originated in oceanic waters of moderate depth, wherever any 



