400 HISTORICAL GEOLOGY. 



When the continuity can be established, the evidence may sometimes lead 

 to unexpected results. For example, it may be found that a coal-bed, 

 followed for some miles to one side or the other, is continuous Avith a shale, 

 and both are actually one layer ; that a sandstone is one with a limestone a 

 few miles off; that an earthy limestone full of fossils is identical with a 

 layer of white crystalline marble in a neighboring district ; or that a fossi- 

 liferous shale of one region is the same stratum with the mica schist of 

 another. 



Precaution 3. — Note whether the strata overlie one another conformably 

 or not, — that is, conformably as regards bedding. 



Precaution 4. — Remember that, where one bed overlies another conform- 

 ably, it does not follow necessarily that these beds belong to consecutive 

 periods, as has been above explained. 



The criterion mentioned, — order of superposition, — unless connected 

 with others, gives no aid in comparing the rocks of distant or disconnected 

 regions. Tor this purpose, other means must be employed. 



2. Color, texture, and mineral composition. — These characteristics may 

 sometimes be used to advantage, but only within limited districts and always 

 with distrust. There were at one time in geology an "old red sandstone" 

 and a " new red sandstone " ; and, whenever a red sandstone was found, it 

 was referred at once to one or the other. But it is now well understood 

 that color is of little consequence, even within a small geographical range. 



Mineral composition has more value than color, especially when it is not 

 one of the common kinds. But it is usually to be disregarded. 



One inference from the mineral constitution of a stratum is safe ; that is, 

 that a stratum is more recent than the rock from ivMch its material was derived. 

 Hence, an imbedded fragment of some known rock may afford important 

 evidence with regard to the age of the containing stratum. But the presence 

 of such a fragment does not prove that a long time intervened ; the imbed- 

 ding may have happened in the same period in which the earlier beds of the 

 formation were made. The beds made and consolidated in modern time are 

 often torn up by the waters and put into new beds in some other place. 

 Coral limestones of recent seas are often conglomerates of the recent coral 

 limestone. Limestone breccia is sometimes formed out of the blocks at the 

 foot of a bluff of limestone from which the blocks had fallen. 



3. Although mineral composition is ordinarily unsafe, it has value when 

 two or more conformable strata of constant mineral characters accompany 

 one another. Such evidences may prove identity for hundreds of miles. 

 The association of schist, limestone, and quartzyte from central Vermont to 

 Connecticut and beyond, with only small gradational changes in each of the 

 rocks, serves to identify the Taconic series through its wide distribution. 



4. Fossils. — The criterion for determining the chronological order of 

 strata dependent on kinds of fossils takes direct hold upon time, and, there- 

 fore, is the best ; and, moreover, it serves for the correlation of rocks all over 

 the world. TJie life of the globe has changed with the progress of time. Each 



