4 STRATIGRAPHICAL GEOLOGY 



that historical geology cannot be defined as a complete History of 

 the Earth ; it would be more correctly described as Scenes from the 

 History of the Earth. 



Nomenclature of Bock-groups. As soon as it was recog- 

 nised that the rocks which compose the earth's crust were not a 

 mere congeries of rock-masses without order or sequence, but that 

 when properly interpreted they arranged themselves in an orderly 

 succession of formations, then it was. seen to be necessary that 

 names should be given to all parts of this wonderful succession of 

 stratified deposits. 



The history which can be read from the " records of the rocks " 

 is a very long one, although, as has already been stated, it is by no 

 means a complete history. Every natural region of the earth's 

 surface has its own geological history, but the same great periods 

 of time can be recognised in all. Geological history, like human 

 history, has its early or pristine times, its mediaeval times, and its 

 later or more modern times ; it may perhaps be said to consist of 

 several volumes, and of course each of these volumes consists of 

 several chapters dealing with so many successive periods. 



Stratigraphical nomenclature is not very satisfactory, for it has 

 been gradually developed in a synthetic manner, and is not the 

 consistent and harmonious arrangement which it might have been 

 had the older geologists possessed our present knowledge of the 

 rocks. In early days the greater rock -groups were generally called 

 formations ; thus we read of the Old Red Sandstone formation, the 

 New Red Sandstone formation, the Carboniferous formation, and 

 the Chalk formation. Afterwards, as a knowledge of their fossil 

 contents increased, the relative time-values of these formations were 

 better understood, and some of them were united with others to 

 form systems, but the number of such systems was not fixed, nor 

 did any definite idea exist as to what entitled a series of rocks to 

 rank & a system. There is still some difference of opinion on this 

 point, but doubtless agreement will ultimately be attained. 



It was also early perceived that the older formations differed 

 very greatly from the later deposits both in mineral characters and 

 in fossil cor>tnts. It was noticed that the oldest rocks were 

 generally crystalline schists, that the next oldest were more or 

 less altered and indurated, while the later were often soft and 

 clearly stratified, and the most recent were usually loose and 

 unconsolidated. Hence Werner in Germany used names which 

 may be translated as equivalent to primitive, transition, stratified, 

 and alluvial rocks, and the French introduced the more convenient 

 terms Primary, Secondary, and Tertiary divisions. 



It was supposed that the rocks belonging to these eras of time 



