292 ELIOT BLACKWELDER 
of ancient pre-Cambrian terranes, or others which are devoid of 
fossils, it may not be practicable to determine how great a thickness 
of the record is lost, and, much less, the time through which the land 
conditions endured. The term “great,” then, has value as indicating 
the geologist’s opinion that the discordance is pronounced and that 
it doubtless implies great loss of record. In this sense it is a con- 
venient word and has been of much service. Nevertheless, for 
the sake of clearness, the three factors should be carefully dis- 
criminated wherever that is feasible, even if their value cannot be 
definitely appraised. 
To show that the stratigraphic hiatus’ is not necessarily a measure 
of the lapse of time during which the unconformity was being made, 
I may cite Le Conte, who says: “Every case of unconformity repre- 
sents a gap in the geologic record at that place..... The loss 
of record may be partly by erosion, but mostly because not written 
at that place.”? Unquestionably the stratigraphic break represents 
a lapse of time not now recorded in that place. But the region may 
have continued to be the scene of deposition during a part of that 
time, and the strata thus formed, and carrying the record, have been 
removed in the ensuing period of erosion. 
By way of illustration we may take two unconformities which are 
somewhat similar as regards the length of the unrecorded interval, 
but are very different in time-value. At Rome, Georgia, Tertiary 
strata rest upon folded Cambrian rocks. In the Bear Lodge Moun- 
tains, northwest of the Black Hills of North Dakota, Tertiary beds 
may also be found upon Cambrian strata at certain points. ‘The 
lost interval in each section is represented by all the strata from late 
Cambrian to Tertiary. In the first case, however, the deposition 
of sediments continued with brief interruptions from Cambrian to at 
least Pennsylvanian times, and then apparently was supplanted by 
erosion from Permian to late Tertiary times. In the second case 
sedimentation persisted until the end of the Cretaceous period, and 
gave way to erosion only during the Eocene period. It is plain, 
therefore, that although the stratigraphic break is nearly identical in 
t By this term is meant the gap in the strata; i. e., where Devonian lies on Cam- 
brian, the stratigraphic hiatus is equivalent to the Ordovician and Silurian systems. 
2 Jos. Le Conte, Elements of Geology, 3d ed. (1893), p. 181. 
