113 



lito to La Junta -was cut by an overflow ot the volcanic dam at a spot dif- 

 ferent from the old exit of the Urique river. Upon this supposition, the 

 placers near Baca were produced by the Chinipas river only. 



The cause of the small volcanic groups outside the edge of the ta- 

 ble-land appears to lie in the existence of a great shearing stress acting 

 on the Earth's crust at this place. The mesa sloping down from 9000 to 

 6000 feet and then plunging abruptly to i iverchannels 1000 feet above the 

 sea, causes a sudden change of load, due to the weight of 5000 feet thick- 

 ness of rock; and, as erosion changes the position of maximum strees, weak 

 places are disclosed, through which any hot fluid matter below is squee- 

 zed out, while the plateau settles down. This action may have been re- 

 peated several times while the plateau was being cut back, thus accoun- 

 ting for the volcanic hills at Topolobampo, and generally along the coast. 



In the clear waters of Topolobampo Bay, pelicans fish day and night 

 unceasingly, turtles bask, and porpoises race by at railway speed, punc- 

 tuating the silence of night with their rhythmical snorts, while hundreds 

 of thousands of sea-birds feed on the sandy beaches of San Ignacio and 

 nest ond islands in the bay, watched over by the paternal care of the Me- 

 xican Government. 



If we depart by sea the last view of the coast is a weird outline of 

 fantastic shapes, distorted by a veil of mirage. 



APPENDIX. 



Generally the highest temperatures were observed in narrow, deep 

 river valleys; the low open plains were cool at night, but had a high tem- 

 perature in the day; the coolest night was April 6, at Orochibo, elevation 

 7575 feet, on the mesa, when there were 19° of frost and thick ice on still 

 water. There was slight cloud at sunset in the plains on February 6, 8, 

 10, 13, and 20, with very brilliant colouring; thunderstorms occurred in the 

 foot-hills on March 19 and 27, with heavy rain and small freshets in the 

 river. At San Bias, on Fehruary 7, there was a dense fog in the morning, 

 with minimum temperature 40° F. No other fog, cloud, or rain was ob- 

 served during February, March, and April. 



There was a gale from the N. W. in the gulf on January 29 and 30, 

 with clear sky, no rain, and a heavy sea; on the coast the wind was fresh 

 to strong from the west on February 1, 2, and 3. Inland it was generally 

 calm, in the foothills at San Juan de Dios, there was a strong, hot, dry 

 wind down the valley on February 26. During March and April a dead 

 calm lay over the mountains. 



Revista[1900-1901J — 15. 



