In this great volcanic area, the lava erupted in a succession of flows. The series of eruptions were separated by long periods of inactivity. During these inactive times, erosion cut valleys and wore down parts of the old lava sheets. This action formed new channels and lower terrain over which succeeding lava flows spread. This process was repeated at least three times. The oldest lavas, which have been exposed by erosion, are found on the tops of the highest mesas. The last series of eruptions created Capulin Mountain; they were ejections mostly of cinders and ash, with less lava flow than in the preceding volcanic activity. These cinder-and-ash eruptions were so recent, geologically, that some of the nearby, bare, steep-sided cinder cones appear as if they had just cooled. The National Park Service has constructed a road up the mountain to the rim of the crater. And from the parking area at the rim, two trails lead to the most interesting points on the mountain. One trail, self-guiding, makes the complete one-mile circuit of the crater along the rim. A guide booklet, keyed to numbered stakes that have been set at places along the trail, explains the features of particular significance—both on the mountain and in the distance. The other trail, less scenic, provides a rare opportunity to see a volcanic mountain from the inside out, for it leads down to the bottom of the crater. [Illustration: View of Chaco Canyon National Monument] Chaco Canyon National Monument Rich in areas of geological interest, New Mexico is equally rich in areas of archeological interest. Four of these are preserved as units of the National Park System. Chaco Canyon National Monument, in the northwestern part of the state, contains more than a dozen large prehistoric ruins and hundreds of smaller archeological sites. These ruins are considered to be without equal north of central Mexico. No other archeological area in the Southwest exhibits such a high development of prehistoric Pueblo civilization. Most of the sites lie within a strip of land about eight miles long and two miles wide, through which Chaco Canyon runs. Although the Monument is relatively isolated, it is open year around and is visited by about 22,000 people annually. The most imposing and best known of the ruins is Pueblo Bonito. Built between 800 and 900 years ago, this 4- and 5-story village covered more than three acres and contained about 800 rooms and 32 kivas, or ceremonial chambers. Archeologist Neil M. Judd, of the National Museum, who conducted some of the late excavations, has said that Pueblo Bonito was the largest apartment house built anywhere in the world prior to 1887, estimating that at one time it housed 1200 people.