1865.] SPRUCE VOLCANIC ROCKS. 249 



13. Leptrena monilifera, M^Coy. 23. Cheirurus clavifrons. 



14. Fenestella assimilis. 24. bimucronatus, 



15. Ptilodictyum dichotomum. 25. Illsenus Davisii. 



16. Theca triangularis. 26. Ampyx tumidus. 



17. Tlieca, ap. 27. Lichas, sp. 



18. Conularia elongata. 28. Harpes, sp. 



19. Holopea concinna, M^Coy. 29. Pygidium, probably of Cheirurus 



20. Modiolopsis Nerei. octolobattis. 



21. Orthoceras vagans. 30. Cythere pliaseolus. 



22. O. politum?, M'Coy. 



2. Note on the Volcanic Tufa of Latacunga, at the foot of Coto- 

 PAXi ; and on the Cangaua, or Volcanic Mud of the Quitenian 

 Andes. By Richabd Spruce, Esq. 



[Communicated by Sir E. I. Murchison, K.C.B., F.E.S., F.G.S.] 



(Abstract.) 



In this paper it was stated that the town of Latacunga (in lat. 1° 

 S., and 9000 feet above the sea) is entirely bnilt of a volcanic tufa, 

 immense deposits of which occur there, that this tufa is due to 

 ancient eruptions of Cotopaxi, and that the smiths of Latacunga 

 and Ambato use it instead of charcoal whenever their stock of that 

 combustible is exhausted. Mr. Spruce then described the tufa as 

 whitish, light, porous, and more or less fibrous. It fuses after 

 having been heated to redness for a considerable time, and on cool- 

 ing becomes a vitreous mass with a glossy white or greenish sur- 

 face. It has then only half its original volume, but nearly the 

 same weight. 



The deposits of volcanic mud called " Cangaua " were also de- 

 scribed. This substance is met with throughout the Quitenian 

 Andes, and the modern towns of Ambato and Riobamba are built of 

 it, though it is said not to constitute a good buUding-material ; it is 

 compact, slightly argillaceous, more or less saline, and yields very 

 slowly to atmospheric agencies or even to running water. The most 

 extensive beds of it, the date of which is known, occur in valleys 

 east of Carguairazo. This mountain was formerly as high as Chim- 

 borazo; but on the night of June 29th, 1699, its hoUow cone fell in, 

 with a shock that destroyed the neighbouring towns, including 

 Ambato, Riobamba, and Alausi, the latter nearly a degree of lati- 

 tude to the southward. Very few of the inhabitants of Ambato 

 escaped ; and scarcely had they gathered together on the ruins of 

 the town when they were driven away by floods of fetid mud, 

 which united just below its site. The river Patate (a tributary of 

 the Pastasa) and the Ambato, which runs into it, were for some 

 weeks blocked up, and their waters spread over the country, forming 

 large lakes ; but they finally re-excavated their original channels*. 

 One valley, however, between Ambato and the village of Tisalco 



* These particulars were obtained by the autlaor from an account of the 

 catastrophe in the archives of Ambato, written by one of the few survivors. 



