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��THE SCIENTIFIC MONTHLY
��large ** entonnoir '^ on the Franco-German line of struggle is shown on Page 429. Judging from some details of remains of passageways shown in the foreground^ its diameter is about 150 feet and depth 50 feet. It was produced by the explosion of several tons of some powerful ex- plosive placed in a chamber at the end of a long tunnel 50 to 60 feet below the surface. The end of the tunnel next the chamber was securely blocked before exploding the mine in order to have the utmost
���Fig. 1. CONTOUB Map and Cboss Section or the Cbatbb of Alchicbica in Pdbbla^
Mexico, attsb Obdombz.
effect of upheaval. The material apparently is compact earth and probably the uplifted material was torn out without much upbending of the sides from which it is detached.
Several years ago Ordonez described some remarkable craters in Mexico which afford some very interesting facts for comparison. One group is on the plateau near Orizaba Peak in Puebla ; the other group is in Valle Santiago. Ordonez regards them as the result of explosions marking the last phase of volcanic activity in the region. The cause was igneous action at no great depth and the locus of explosion was determined by some superficial influence.
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