Subsurface Labobatoey Methods 91 



(2) one or more species or genera, ^3j general assemblages, (4) relative 

 abundance of one or more than one species or genus, (5) ratio of various 

 species and genera, (6) evolutionary development, and (7) faunal 

 sequence. 



The recognition of key species requires considerable detailed work. 

 The index value of these forms must be repeatedly tested and compared in 

 several stratigraphic sequences. For example, it required the writer two 

 vears to determine ten key species from a 375-species fauna of the Middle 

 Tertiary deposits of central Sumatra. A given species may have an index 

 value in one area, whereas its value as such may be slight or even worth- 

 less in adjacent areas. The responsibility to recognize and determine 

 these peculiarities rests with the micropaleontologist 



It is general laboratory practice to develop a species type or control 

 set for reference and comparison. Type species are either identified spe- 

 cifically or are given work numbers. The latter procedure is most applica- 

 ble when a reference is not available for establishing species determina- 

 tions. 



In working foraminiferal faunas and interpreting their significance, 

 it is necessary' constantly to keep in mind the facies concept. Natland ^ 

 demonstrated that recent foraminiferal assemblages off the coast of 

 southern California differ in composition from shallow to deep water. 

 He recognized a similar faunal sequence vertically in the Pliocene section 

 of the Ventura Basin, California. The significance of this relationship 

 exemplifies that these various assemblages have transected time horizons, 

 a characteristic that many faimas undoubtedly possess. This problem con- 

 stantly confronts micropaleontologists when long-range correlations are 

 based on benthonic assemblages. Unlike faunas may be of similar age, 

 although similar faunas may not possess the same time value. 



LoA\Tnan ^ in his studies of the distribution of recent Foraminifera of 

 the Gulf of Mexico has shown the extreme lateral variation of this group 

 from fresh-water to brackish-water to open-sea en\"ironments. The object 

 of this investigation was to improve "our ability to use fossil foraminifera 

 as criteria of depositional environments, and, second, to help disentangle 

 environmental and evolutionary factors in the three-dimensional dis- 

 tribution of fossil faunas." 



A unique method employed by the Bataafsche Petroleum Maat- 

 schappij (Royal Dutch-Shell) in preparing micropaleontologic facies logs 

 is described by Ten Dam.- This method is based primarily on the quanti- 

 tative occurrence of certain genera or groups of genera of Foraminifera. 

 Ten Dam summarizes the general procedure as follows: 



The microfauna of each washed sample is divided into benthonic and 



' Natland, M. L., The Temperature and Depth Distribution of Some Recent and Fossil Foraminifera 

 in the Southern California Region: Scripps Inst. Oceanography Boll., toL 3, no. 10. pp. 223-230. 1933. 



^ Lowman, S. W., Sedimentary Facies of Gulf Coast: Am. Assoc. Fetrolenm Geologists, voL 23, no. 

 12, pp. 19391997, 1949. 



* Ten Dam, A.. Micro paleontological Fades-Logs: The Micropaleontologist, toL 1, no. 4, pp. 13-15, 

 Oct. 1947. 



