RECONNAISSANCE OF MINDANAO AND SULU : III. 381 



rate the corrosive and dissolving power of the water which pours in with great 

 velocity upon the place which they have chosen so that a kind of shaft is rapidly 

 sunk till it reaches the barren ground upon which the alluvion rests. At the same 

 time, while the water is loosening up the pieces and lumps, the workers remove 

 the mud thus produced and throw it out continuously upon the surface, in order 

 that the rich sands may be deposited and the clay and barren matter may be 

 washed out by the stream. In this manner, the advancing excavation reaches a 

 depth equal to that of the thickness of the alluvion, pushing gradually forward 

 horizontally along the course of the canal which leads to the reservoir. In order 

 that the advance may be slow and thus permit of exploiting the placer in all 

 directions they open to the right and left of the main canal a number of cross 

 cuts, which conducting the water from the sides, make other lateral workings 

 which as they advance give a more or less circular form to the whole. 



The large boulders encountered in the lower part of the alluvion they sort out 

 by hand and use for making the walls of the run-off after it has been used. The 

 excavation which results, whether consisting of one trench or of several, is called 

 in the district ianlasan. This part of the work has certain similarities to the 

 method which the washers with the gandingas use in the Province of Mediodia in 

 Spain, in the sluice boxes called roUos or arollos. Nevertheless, the banlasan is 

 larger and less skillfully made than the rollos, and they work with a greater 

 quantity of water and with a greater velocity. 



Once or twice a week, depending upon the richness of the placer, when the 

 upper part of each trench is sufficiently full of rich sands, they close the gates 

 of the reservoir and give each trough a chance to dry out. They then take these 

 sands to the bank of the reservoir and by means of the bilingan already described 

 in the river washings, concentrate and separate the quartz, zircon, etc., from the 

 fairly clean gold dust and apportion as indicated above. 



To summarize, this pi'oceeding has four phases distinct as to order and object. 

 ( 1 ) Investigation of the deposit by means of tujuhs, not always necessary, as for 

 example when by other indications or by known signs they know beforehand that 

 the placer is rich in the part they expect to work. (2) Preparation for the 

 work by the construction of a canal, reservoir, and all the other work. (3) 

 Concentration of the alluvial ground or the exploitation proper of the deposit, by 

 means of the work in the banlasan. (4) The cleaning up (depuracion) of the 

 sands by hand washing with the hilingan finally obtaining the gold free. 



In another paper published about the same time as the report just 

 referred to/- Abella gives some further information regarding the geology 

 of this country. He refers to a number of formations which I shall take 

 up briefly. 



He noted a considerable limestone formation which apparently is over- 

 laid by a conglomerate with a calcareous matrix. This conglomerate he 

 calls gonfolita and above this gonfoJita he encountered a yellow marl, 

 fairly loose and quite like that which covers the slopes of the divide 

 between the Cagayan and Iponan Elvers. This marl is .probably the 

 result of the weathering of the matrix of the conglomerate. The pebbles 

 of this conglomerate, according to Abella, are serpentine, argillaceous 

 slates, and trachytic rocks of many types. He also found an arkose 



" Itinerarios Geologicos. Observaciones tornados al paso en los viajes hechos 

 & las comarcas aurlferas de Misamis. 



