UNSTRATIFIED CONDITION. 117 



of the Triassic, which may add a few thousands to the amount. 

 In Virginia the thickness is still greater ; but no exact estimate 

 has been made. In Indiana and the other States west it is only 

 4000, although extending, as in Pennsylvania, to the top of the 

 Carboniferous. The greater part of the continent of North Ame- 

 rica east of the Mississippi is destitute of rocks above the Carboni- 

 ferous. 



In Europe the rocks of the later periods are far more complete 

 than in North America, while the older also, according to the 

 estimates stated, exceed the American. In Great Britain the 

 thickness to the top of the Carboniferous is over 60,000 feet, and 

 from the Carboniferous to the top of the series little less than 

 10,000 feet more. This amount is the sum of the thickest deposits 

 of the several formations, and not the thickness observed in any 

 particular place. 



2. UNSTRATIFIED CONDITION. 



128. The larger part of the crystallized rocks are sedimentary 

 rocks altered or crystallized by heat or other means ; 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 deposit, 

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

 they ever were. 



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

 cation 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 now their ajDparent con- 

 dition. Granite and syenite are unstratified rocks of this kind. In 

 much granite there is no lamination, no arrangement of the con* 

 stituent minerals in parallel planes, no evidence of subdivision 

 into layers. But even this true granite, a few miles off, may become 

 a schistose or gneissoid rock, and, a short distance farther on, 

 by gradual transition, a 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 dolerite which range in lofty 

 masses over many districts — as the Palisades on the Hudson, Mounts 

 Tom and Holyoke and other trap ridges of the Connecticut valley, 

 the trap of the Giants' Causeway and of 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 



