XX. 



fauna includes the present animal life of that region. Simi- 

 larly the Hamilton fauna is the sum of the animal life which 

 existed during that period. We may speak of the fauna of a 

 stratum, as for instance the " Encrinal limestone fauna," or 

 the " Spirifer sculptilis fauna," or the fauna of a bed viz : the 

 " Demissa fauna." 



Methods of Study. — In beginning the study of the stratified 

 rocks of this region, it is highly desirable that a stratum 

 be selected which may be used as a datum plane, with 

 reference to which the position of all the beds may be ascer- 

 tained. There are two such reference strata in this region, 

 both of which, on account of their great areal extent over 

 Western New York, will also serve in the correlation of the 

 strata of the Eighteen Mile Creek region with those of more 

 eastern localities. These strata are the Styliolina limestone, 

 which here forms the base of the Upper Devonian, and the 

 Encrinal limestone, which separates the Moscow and Hamil- 

 ton shales. The first of these is seen in seven of the eight 

 sections in Eighteen Mile Creek, and again in the first of the 

 South Shore Cliffs. The Encrinal limestone is first exposed 

 in Section 5 in Eighteen Mile Creek, and appears in all the 

 sections below that one, as well as in the cliffs on both sides 

 of the mouth of Eighteen Mile Creek. While therefore, the 

 Stvliolina limestone forms a reference plane for the upper 

 strata of this region, the Encrinal limestone becomes a con- 

 venient datum plane for the lower beds. 



A third stratum which may be used as a reference plane in 

 this region, is the Strophalosia bed, which lies fifty feet 

 below the Encrinal limestone, and is considered the top bed 

 of the Marcellus stage. This bed is exposed in Avery's 

 Creek, and in Erie and Athol Springs Cliffs. 



The study of the several cliffs is best undertaken in the 

 order in which thev are described in chapters one and two. 

 The following itinary is suggested: Leave the train at North 

 Evans station, and descend into the gorge by the steps of the 



