208 REPORTS ON INVESTIGATIONS AND PROJECTS. 



original data, the raw material, systematically. Gaps in the evi- 

 dence may later be filled in, reasons for controverted inferences may 

 be called for, revision of views may become desirable, or the investi- 

 gation may pass into new hands. In any case the original assemblage 

 of facts should be well ordered. 



Interpretation. — Interpretation is deciphering the record of geo- 

 graphic conditions embodied in sedimentary strata and associated 

 rocks, according to the processes of epeirogeny, erosion, and sedi- 

 mentation. We proceed from the certain to the uncertain. Thus, 

 given the area of occurrence of a sedimentary formation, we know 

 that there was a coextensive water body ; given a change in thick- 

 ness and constitution, a change which takes place in a definite direc- 

 tion, we may state in what direction lay the source of the sediment, 

 a land area ; given those peculiarities which are characteristic of the 

 littoral, we may approximately delineate the position of the shore 

 zone, and thus delimit land and sea for that area and that epoch. 

 Thus far we may often proceed with reasonable certainty, since we 

 deal with a definite condition which admits of no alternative, and 

 thus far, but no further, geologists have usually gone ; for they com- 

 monly leave untouched the problems of physiography and climate 

 during a specific epoch, or the sequence of migration and fluctuation 

 of conditions from age to age. 



In reference to physiography we may state as our guiding prin- 

 ciple that the volume of sediment deposited in a given area during 

 a certain epoch is a function of the area and altitude of the land from 

 which it came. For instance, strata of the Devonian age in Alabama 

 are but 30 feet thick ; in Pennsylvania strata which correspond in 

 age are 10,000 feet thick. There is evidence that the shore was not 

 notably farther from the one than from the other deposit ; but 

 clearly in one region the nearby land, though possibly extensive, was 

 low and yielded almost no sediment, whereas in the other district it 

 was not only extensive, but also elevated. In Pennsylvania and 

 adjacent States the character of the sediments, that of sandy alluvium, 

 bears out the inference that they came from a mountainous region, 

 whence large rivers flowed to the sea, and as the volume of strata is 

 equal to that of a Sierra Nevada, we have a suggestion of a vanished 

 mountain range that rose somewhere along the eastern border of the 

 continent, probably where now is the western Atlantic. 



A contrasting physiographic inference is that which relates to both 

 North America and eastern Asia during the Cambrian period. At 

 the base of the Cambrian series in each country is a stratum of 

 mechanical sediment, spread by the sea which swept over thousands 



