44 



istae, establisliing its Middle Cretaoeous age. The railway raiis over this 

 limestone from the HOtli kilometre across Oiuzaba as far as the 180t,]i ki- 

 lometre iiear the statiou oí' Esperanza, after wich the country is completely 

 covered by volcanic ejecta. 



Upon this calcareous bed, sink-holes swallowing up rivers are of very 

 frequent occiirrence. The SiDanish narae for a funnel-sapped depressioii 

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

 this nanie. These sumideros correspond to aii extensive system of subter- 

 ranean channels: the deepest are still used by undergroand streatns, 

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

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

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

 this class, 



Location. — The mouth of the cave is located amidst steep slopes 900 

 metres east of the station at Atoyac, about 70 metres beloAv 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 snmmer of 1906 by 

 Señor Sánchez when hunting big game among these .jrecipices. During 

 the follomng autumn the writer liad an opportunity of visiting it with 

 Profes^or C. de la Torre (Ha vana University), and of making a rapid surv- 

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

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

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

 strike. A glance at the plan of the cave shows that the snccession of chan- 

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

 The main passages ran in a íiorth to south direction, according to the 

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

 verse corridors corresponding to the joint planes and obsequent to the 

 direction of the strata. 



The features of tlie interior were briefly described and accounted for. 

 A transverse corridor is partially closed by a high ridge which has been 

 probably formed by the blocks of limestone falliiig from tlie roof, damming 

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

 Near the top of the cave is a series of narrow tortuous passages, recalling 

 to the mind the worm of a still. They are superposed one above another. 

 and suggest the progi'essive tunnelling down of the waters. AU these 

 narrow tunnels run to the bottom of a vertical shaft, which the wiúter was 

 not able to explore; but a constant current of fresh air (temperature 20° 

 centigrade) gives evidence of a direct communication with the surface of 

 the soil. It corresponde in all probability to a chimey by which the surface 

 waters were formerly engulfed. 



