302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



are working in the United States alone, and that it costs at least 

 $30,000 to equip a crew and $3,500 a month to run it. Crews with 

 equipment may be hired at an inclusive charge of $9,000 per month. 

 No companies are organized for this work in Canada, but some 

 United States companies and also some from Germany have oper- 

 ated here. 



It has been a long story, with many very summary and sketchy 

 flights over details. We have seen how seismology penetrates to 

 depths of a mile or two so quickly and surveys wide areas so rapidly 

 that the expensive wielding of pick or drill need not be resorted to 

 without some reasonable hope of reward. Aftershocks permit the 

 extension of such methods to the bottom of the crust, which we have 

 found to be of varying stability, varying thickness, and varying 

 structure. The work on the crust permits us to arrive at a time- 

 distance curve for a hypothetically stripped earth, after which the 

 Herglotz-Wiechert method enables us to obtain a velocity-depth 

 curve for the mantle. With these data at hand, we may compute the 

 arrival times for various complex waves which should record at the 

 surface, thus checking the previous deductions. We know some 

 things about the core but it still remains to a large extent an un- 

 solved problem — by no means the only one in seismology of c "lurse. 



So, step by step, year by year, slow but steady progress is eing 

 made, by cooperative international effort, toward a more accurate 

 knowledge of earth structure. This is one of the main reasons for 

 studying earthquakes, whether in a seismic region or in relatively 

 quiescent Canada. 



