GEJSliEAL OBSEEVATIONS. 



CHAPTER I. 



GENERAL OBSERVATIONS UPON THE COLLECTIONS AND 

 THE PERIODS THEY REPRESENT. 



The critical investigation of fossils collected in newly-explored regions, 

 distant from those in which the stratigraphical relations of the rocks have 

 been accurately determined, especially if the relative position of the strata 

 of those regions has been obscured by disturbance, it is often difficult and 

 sometimes impracticable to assign each collection to its proper geological 

 horizon. The difficulty is greatly increased to the paleontologist if he must 

 prepare his report before those of the field-geologists are available, or if he 

 has not himself visited the regions in question and obtained by personal 

 observation a knowledge of the strata and their relative positions. 



This difficulty has been met in the present instance, so far as any person 

 could do it, by Mr. G. K. Gilbert, geologist of the expedition, by whose 

 patient and clear oral and epistolary explanations I have greatly profited. 

 If the collections were abundant in every case, it would be a comparatively 

 easy task to assign them to their proper horizons ; but it is often necessary 

 to make some such assignment of a meager and uncharacteristic collection, 

 or to pass it by in silence. This alternative has been presented to me in 

 some instances by the collections that form the basis of this report ; and I 

 have chosen to make some assignment of them in all cases according to the 

 best evidence presented, indicating at the same time such doubts as I have 

 entertained. 



The accompanying table of the subdivisions of geological time, which 

 are more or less familiar to American geologists, is introduced here for the 

 purpose of showing at a glance what periods and epochs are represented by 

 the fossils of the collections, as well as the relative positions of the strata 

 from which they were obtained. The names of the periods ai'e those used 

 by Dana in his new Manual of Geology (1874), and will be found to differ 

 slightly from those formerly in use. 



