They also reveal density, and hence the types of rock. Moving cross-country, the machines make subterranean profiles known as seismic lines. Seismic shots with explosives have been used for years in the search for oil. Alaska is crisscrossed with all but indelible “seismic” disruptions of tundra. The reserves of Prudhoe Bay were discovered in this manner. Dynamite in the populous East could irritate the public, so the universities and the U.S.G.S. use a behemoth called Vibroseis to shiver the timbers of the earth. One of the first discoveries the vibrations reported was that the Brevard Zone is relatively shallow and the crust below it is American rock that does not in any vague way reflect a continent-to-continent suture. Africa was co-working space groningen nowhere in the picture. The Brevard Zone proved to be the toboggan-like front end of a large and essentially horizontal thrust sheet. Plate-tectonic theorists accommodated this news by moving the suture fifty miles east. The new edge of Africa was under Kings Mountain. Seismic shots took the stitches out of Kings Mountain. ‘When we got the data for the Brevard” -as Leonard Harris liked to tell the story-“they pushed the suture to Kings Mountain, and when we got data for that they said the suture must be under the coastal plain, and now that we are getting data for the coastal plain they say it must. be in tl1e continental shelf. Well, we’ve got data out there, too.” Up and down the Appalachians, wherever such data were collected, thrust sheets were seen to have moved in a northwesterly direction, and much of the thrusting had never been suspected before. Conventional co-working space apeldoorn thought had been that the old rock of the Green Mountains, the Berkshires, the New Jersey Highlands, the Catoctin Mountains, and the northern Blue Ridge was in place, firmly rooted-autochthonous, as geologists are wont to say.