44 



istae, establishing its Middlo Cretaf.eous age. The railway riins ovev this 

 limestone from the 80tli kilometre across Orizaba as far as the ISOtli ki- 

 lometre near the station of Esperaaza, after wioh tlie country is completely 

 covered bj^ volcanic ejecta. 



Upon this calcareous bed. siiik-holes swallowiiig up rivers are of very 

 frequent occurrence. The Spanish uame for a funiiel-sapped depressioii 

 of this kind is sumidero, and one of the railway stations has been called by 

 this iiauíe. These sumuJcros eorrespoud to an extensive system of siibter- 

 ranean ehannels: the deepest are still used by undergroaiid streams, 

 whereas the higher ones are mostly dry, and ha ve reached a state of oíd 

 age, characterised by the depositiou of sinter and the formation of stalact- 

 ites tending to obstruct them again. The cave of Atoyac is an instance of 

 this elass, 



Location. — The mouth of the cave is located amidst steep slopes 5)00 

 metres east of the station at Atoyac, about 70 metres below the railway, 

 and 26 metres above the present level of the river flowing at the bottom 

 of the gorge. The opening was discovered during the summer of 1906 by 

 Señor Sánchez when liunting big game araong these .ireeipices. During 

 the follo^dng autumn the wi-iter had an opportunity of visiting it with 

 Profes^or C. de la Torre (Havana University), and of maldng a rapid surv- 

 ey of it. The entrauce is partielly closed by the fall of debris. Tlie strike 

 of the cretaceous limestone at this point is north to south, the dip is about 

 75 degrees east, and the joiuting is aproximately perpendicular to the 

 strike. A glance at the plan of the cave shows that the succession of ehan- 

 nels and chambers is not random, but presents a rectangular arrangement. 

 The inain passages liin iii a north to south direction, according to the 

 strike ; the may be called subsequent. They are counected by smaller trans- 

 verso corridors corresponding to the joiiit ])lanes and obsoquent to the 

 direction of the sti'ata. 



Tlje features of the interíor were briefly desóribed and accounted for. 

 A transverso corridor is partially closed by a high ridge wliich has been 

 prpbably formed by the blocks of limestone falling froni the roof, damniing 

 back the water and slowly covered by the sinter deposited by the cascade. 

 Near the top of the cave is a series of naiTow tortuous i>assages, recalling 

 to the inind the worin of a still. They are superposed one above another. 

 and suggest the progressive tunnelling down of tlic waters. All these 

 narrow tunnels run to the bottom of a vertical shaft, wliich the writer was 

 not able to explore; but a constant current of ft-esh air (temperature 20*^ 

 centigi-íule) gives evidence of a direct communication with the surface of 

 the soil. It corresponde in all probability to a cliimey by whicli the surface 

 wáter.s were fonnerly eugulfed. 



