858 TEXTBOOK OF ZOOLOGY 



mary will be given of the particular methods which apply to this 

 problem. Two of the methods are here described diagrammatically. 

 First, is the device of bracketing or use of associations, the term 

 being used here with much less exactness than in ecology. Several 

 long-lived species are represented by the symbols (triangles, squares, 

 circles) , Each is a species which has a thousand feet or more of ver- 

 tical range, and the ranges are known. In a well sample the appear- 

 ance of no single one of these species would be of much use to the 

 paleontologist. Certain associations might indicate limited horizons 

 upon which the petroleum geologist could carry his structures from 

 well to well. 



Using this method of bracketing on the larger fossils in the Lower 

 Cretaceous period of North and West Texas during the past twenty 

 years the writer has been able to locate for engineers, where highways 

 should run to have the greatest volume of the cuts in soft marls. He 

 has also, on many occasions, been able to foretell for builders the 

 exact depth to which foundations would have to be carried for large 

 buildings, dams, and other masonry structures. 



The greatest development of this method has been in the work 

 with microfossils. As long ago as 1922, three young women,* work- 

 ing in the laboratories of oil companies in Houston, astounded a 

 large audience of petroleum geologists by demonstrating beyond 

 question that they could locate horizons as narrow as twenty feet. 

 This was done in the soft Tertiary sediments of Southeast Texas, 

 where geologists, up to that time, had been satisfied with divisions 

 running from five hundred to a thousand feet. 



The second methodf is based on the fact that many marine or- 

 ganisms suddenly disappear from the seas which they have long 

 occupied. This extinction is caused by a change in food supply, a 

 change in temperature or other factor, or by the appearance of new 

 enemies. A rapid extinction of a species, in terms of the record in 

 the sediments, may be said to be almost instantaneous. In other 

 words, the end of a species is more likely to be startlingly abrupt 

 than is its beginning. This is true, notwithstanding the fact that 

 with favorable conditions a species can populate an area of land 

 or a portion of the sea in a very short time. An everyday example 



•Misses Hedwlg Knicker and Alva Ellisor and Mrs. P. L. Applin. 

 tFlrst published by Mr. N. L. Thomas, paleontologist for the Pure Oil Company, 

 ftltliough not claiming- the method as original. 



