88 WARREN N. THAYER 



water is not derived directly from rainfall, but from a subterranean 

 sheet flowing under the porous volcanic soil. The slopes of the 

 craters, both interior and exterior, are frequently covered with 

 green, cultivated fields. 1 These craters are generally regarded as 

 having been produced by explosions caused by steam generated 

 when superficial water came in contact with heated rocks below. 2 

 The basis of this opinion is the fact that the stratified basaltic 

 flows and beds of tuff forming the plain in which these craters are 

 discovered have not been disturbed profoundly, the eruptions 

 appearing to have been strictly local. 3 



The pit craters of Pueblo are apparently similar features. 

 These occupy the centers of cones which rise with very gentle slopes 

 to slight elevations. They are from 3,000 feet to a mile in diameter, 

 and average 150 feet in depth. On the floors of the craters are 

 lakes of fresh water, and their walls show plainly the stratified 

 volcanic beds of which the plain on which they stand is composed. 

 In places are to be observed breccia-filled stream channels, buried 

 under the eruptives which built the cones and formed the craters. 4 

 Their genesis is no doubt similar to that of the cinder cones described 

 above, though no less an able geologist than Ordonez states that 

 they seem unduly large for the explosive force which is generally 

 regarded as having produced them. 5 Both types of phenomena 

 are relatively recent features of the topography, and probably 

 belong to the second period of vulcanism, to be discussed later. 



The important drainage systems of this province are those of 

 the Rio Grande de Santiago, the Rio Lerma, and the Rio Balsas 

 of the Pacific slope, and the Rio Papaloapam with its tributaries 

 and a few small streams of the Atlantic slope. The Rio Lerma is 

 really the upper course of the Rio Grande de Santiago, which is 

 divided about midway in its course by Lake Chapala. The two 

 streams and the lake together form a watercourse about 800 miles 

 in length, the largest in Mexico, if we omit the Rio Grande del 

 Norte, having a drainage basin of about 60,000 square miles. In 



1 Ordonez, Guides des excursions, 10th Inter. Geol. Cong. Mex., iqo6, No. 14. 



2 Hovey, Bull. Am. Gcog. Soc, XXXVIII, 73°- 



3 Ordonez, loc. cit. 4 W. M. Davis, Science, N.S., XXVI, 226. 

 5 Ordonez, Inst. Geo!., Mex., Los Xalalpasgos del Estado de Pueblo. 



