XV. 



formation depending on the physical conditions which 

 existed during the corresponding period of time at the place 

 where they occur. On this account rock groups commonly 

 receive local names, such names being taken from the locality 

 where the group is best developed, or where it was first 

 studied. A number of local rock groups, known by various 

 names, were deposited during each geological period, each 

 group characterizing a different locality, and indicating 

 different physical conditions during the time and at the place 

 of its deposition. For purposes of correlation it is desirable 

 to have one name to which the local groups can be referred, 

 and such a name must be of general applicability. None of 

 the local group names can be selected, no matter how much 

 priority any one of them may have. For example: the 

 Huamampampa sandstone of Bolivia was probably de- 

 posited while the Hamilton sediments were accumulating 

 over New York, but the Huamampampa sandstones are not 

 Hamilton. They are Middle Devonian just as the Hamilton 

 sediments are Middle Devonian. It is therefore proposed to 

 use the terms Lower, Middle and Upper to designate a three- 

 fold division of each rock series deposited during the 

 corresponding geologic era. Thus the Devonic series, 

 built up during the Devonic era, is divisible into three 

 groups, the Lower, Middle and Upper Devonian groups, 

 which were deposited respectively during the Eo, Meso and 

 Neodevonian periods. This division may seem somewhat 

 artificial, especially as some rock series are divisible into 

 more or less than three groups in different localities. Thus 

 in Tennessee the whole of the Devonic series is represented 

 by the Chattanooga black shale, which in places is only 

 twelve feet thick and shows no subdivisions. But in those 

 twelve feet of shale are probably included the Lower, Middle 

 and Upper Devonian groups. One division may be unrepre- 

 sented, as is probably the case in the Devonic or Old Red 

 Sandstones of Scotland, where only a Lower and an Upper 

 group are recognized, the place of the Middle group being 



