UNSTRATIFIED CONDITION. 107 



tinental idiosyncrasies, and on the other to migrational distribution, 

 which is always to be carefully considered. 



Such facts do not lead to any doubt as to conclusions based on the 

 general range of types characterizing an era. Should a trilobite be 

 hereafter discovered in any Cretaceous rocks of the world, it would 

 lead no one to suspect those rocks to be Paleozoic ; because the associ- 

 ated species would unquestionably be true Cretaceous fossils. 



2. UNSTRATIFIED CONDITION. 



The larger part of the crystallized rocks were once fragmental 

 rocks, and have been altered, that is, are metamorphic rocks (p. 66) ; 

 and they are, therefore, not true examples of unstratified rocks. In 

 general, they still retain the lines of deposition distinct. When gneiss 

 and mica schist are found in alternations with one another, it is plain 

 that each layer corresponds to a separate layer in the original de- 

 posit ; and the beds, although crystalline, are still as really stratified 

 as they ever were. 



In some metamorphic rocks, however, the appearance of stratifica- 

 tion is lost ; and such may be properly said to be unstratified. Yet it 

 should be understood that the name does not imply that they never 

 were stratified, but that this is not now their apparent condition. Gran- 

 ite and syenyte are unstratified rocks of this kind. In much granite 

 there is no lamination, no arrangement of the constituent minerals in 

 parallel planes, no evidence of subdivisiou into layers. But even this 

 true granite, a few miles off, may become gneiss in which a schistose 

 structure is very distinct. 



Examples of the unstratified condition are common among true 

 igneous rocks. The ridges of trap or doleryte which range over many 

 districts — as the Palisades on the Hudson, Mounts Tom and Holyoke 

 and the other trap ridges of the Connecticut valley, the Giant's Cause- 

 way and Fingal's Cave — are some of these examples. The rocks 

 were melted when they came up to the light through fissures ; and they 

 now stand without any marks of stratification. The sketch on p. 108 

 represents a scene among rocks of this kind in Australia. The dome- 

 shaped masses of trachyte, in some regions of ancient volcanoes, and 

 the interior mass of many great volcanoes, — sometimes exposed to 

 view through rendings of the mountain or denudation by water, — 

 are also examples. But the ordinary outflows of liquid rock from vol- 

 canoes usually produce layers, which are covered afterward by others 

 in succession ; and volcanic mountains, therefore, have to a great ex- 

 tent a stratified arrangement of the rock-material, and not less per- 

 fectly so than bluffs of stratified limestone. Moreover, the same rock 



