«. 
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36 EUGENE McDERMOTT 
depth determinations opposite it on the cross section. The two re- 
flections, Pecan Gap and basal Austin member, are marked on the 
records and the datum values shown. These datum values are shown 
to scale on the cross section. 
Due to the greater depth of the Austin, the reflection from it 
should be smaller in amplitude than that from the Pecan Gap. Fora 
reasonably large Pecan Gap reflection the Austin reflection amplitude 
NAVARRO COUNTY - TEXAS 
ABSENOMETERS —_ EDT W FEET 
N\ 
1) 


Fic. 1.—Typical reflection record showing relation of reflections to geologic section. 
is almost too small to use. It is, therefore, general practice to take 
several records with different dynamite charges. In this way both 
large- and small-amplitude records are obtained for the two reflec- 
tions. The small-amplitude Austin reflections shown in Figure 4 have 
been further substantiated by larger amplitude records. 
Attention is called to the absence of the Austin reflection on record 
F.P.210. It so happens that the normal reflection point on the Austin 
falls in the Austin fault plane. In view of the extraneous disturbance 
in this record at the time of normal occurrence of the Austin reflec- 
366 
