y\6 EDWIN C. ECKEL 



then it is obvious that the Hoosac schist is simply an equivalent 

 of the Berkshire schist, while none of the Stockbridge limestone 

 is represented by any of the schist east of the mountain. On 

 any hypothesis, therefore, the formation name "Hoosac schist" 

 is apparently untenable. 



Case J. — The third example cited by Mr. Willis is that of a 

 shale which varies horizontally in such a manner as to become 

 differentiated into several superposed members, one of which is 

 a similar shale, the others limestone or sandstone. His decision 

 is that in such a case all the shale can be given the same forma- 

 tion name, while the new members (limestone, sandstone, etc.), 

 are to be given distinctive names ; but that, in case it be desira- 

 ble to refer to the group as a whole in some area where it is thus 

 differentiated into several members, the geographic name applied 

 to the shale alone cannot be applied to the entire group. 



The last three cases cited by Mr. Willis present instances of 

 subordinate parts or local developments (of the lithologic unit) 

 which, though too small for mapping, are of interest in discus- 

 sion. To such sections of the unit the terms "member" or 

 "lens" may be applied. This method of treatment and termi- 

 nology, if followed strictly and uniformly, would seem to be 

 entirely satisfactory. 



SUMMARY. 



It will be seen that the essential character of the proposed 

 unit is that it is uniform, or uniformly variable, in lithologic 

 character. Both its upper and lower limits and its lateral boun- 

 daries will therefore by marked by lithologic differences. So 

 long as the formation is geographically continuous, no other 

 criterion is necessary or even admissible. 



In the case of discontinuities in outcrop, caused by the pres- 

 ence of bodies of water, or of intervening areas of other rocks 

 or superficial deposits, the question will arise as to the treatment 

 in mapping of two distinct bodies of rock, i'dentical lithologically. 

 In this case the aid of stratigraphic association or contained 

 fossils may be invoked. 



In regard to sharpness of definition, ease and accuracy of 



