58 



NOTES ON THE 



assume tlie diameter of the bottom of the crater, including the debris on which many 

 of the falling stones remain — (the horizontal part of the bottom has a diameter of 

 only 750 feet)— to be 1200 feet, the surface will be 1,130,000 square feet, and the 

 quantity of stones falling annually at the rate of 2000 cubic feet for a day, amounts 

 to 730,000 cubic feet, so that, if the bottom was covered uniformly, it Avould rise about 

 0.65 of a foot in a year. But as the stones do not reach the middle, the sides or 

 debris must increase considerably more rapidly. This would indicate that the crater 

 must fill up, and the height of the cone sensibly decrease within a thousand years 

 if the volcano should remain inactive. The diminution of depth will, however, 

 be less and less every year, in proportion as the upper part of the crater expands 

 and the depth diminishes. (Figs. 3 and 4 exhibit a section and plan of the summit ) 



Fig. 3. 



SECTION THROUGH A— 3. 



Fig. 4. 



SUM M IT AN D CRATER OF POPOCATEPLTL 

 Scale — ranooo. 



Besides stones of smaller sizes a large quantity of sand is continually falling 

 which tends to fill up the empty spaces left between the larger stones. It is set 

 in motion by the wind, which generally blows very fresh on the summit, and by 

 fixlling stones. Very little is required to disturb the sand, as it is deposited on the 

 steep walls of the crater. 



After being detached from the rope, and while continuing the descent of the 

 talus, we were several times compelled to take refuge from the felling stones 

 behind a large boulder or a vertical wall. In January and February the debris 

 was nearly clear of snow, but in June we found it covered with a thick layer, the 

 surface of which was so hard that we were obliged to cut steps in it with a 



