Henry Shaler Williams. 683 



formation has its own distinct set of organisms. To 

 Professor Williams, more than to any other American 

 stratigrapher, de we owe our present marked insight into 

 the fact that the marine faunas of the past have shifted 

 about so much that only the expert of many years' expe- 

 rience can discern in them their true correlation values. 



The mind of Professor Williams was distinctly analy- 

 tical, philosophical, and cautious — possibly over-cau- 

 tious. He loved to pick out the parts of a problem and to 

 define them. The components of a series of faunal 

 assemblages were examined in the greatest detail, not 

 only as to their minute changes in the acquisition or loss 

 of characters, but as to their numerical presence as well. 

 On the other hand, he must at times have been lost in the 

 labyrinth of observed detail. All of these studies he 

 thought necessary to find out, both how the assemblages 

 change so that the same congeries indicate a given 

 geologic time, and why these changes and shiftings 

 occur. 



The leading line of study with Professor Williams was 

 that of the Devonian of America east of the Mississippi 

 River, and chiefly of the New York, Maine, and the Appa- 

 lachian areas. Here he sought to learn what were the 

 successive faunas in a given section and how the species 

 and their assemblages differed among themselves over a 

 wide geographic area. He therefore studied the stratig- 

 raphy in detail, and collected the faunas bed by bed along 

 ten or more parallel meridians near enough to one 

 another, in the states of New York and Pennsylvania, to 

 make it possible to compare the corresponding zones of 

 the various formations studied. This is the Williams 

 method of stratigraphic study, and one of which he 

 appears to have been the inventor. In this way, he 

 proved "that the composition of a fossil fauna changes in 

 passing geographically from one place to another. Upon 

 tracing single species across these sections, it was 

 learned that the mutation of the species not only may be 

 recognized on passing vertically upward through a con- 

 tinuous section, but that the more direct line of succession 

 was often deflected laterally so that the immediate suc- 

 cessor of a particular fauna of one section was found not 

 directly above it in the same section, but at a higher 

 horizon in a section ten or twenty miles distant. This 

 shifting of faunas was taken as actual evidence of migra- 



