IX, A, 2 Pratt: Geology of Cement Materials 159 



primary rocks with which the elastics are associated. Abella 

 spoke of these rocks as "tobas" (tuffs), a term which he seems 

 to have used to denominate secondary rocks closely associated 

 with an igneous type from which they were derived, partly 

 by erosion and partly perhaps by residual decomposition. 



Samples 48 and 49 represent earthy clastic rocks of nonuniform 

 grain size, from different horizons in this formation. They 

 were obtained on Pandan River about 6 kilometers inland, from 

 rocks which are imperfectly bedded, much crumpled, and slightly 

 schistose in some exposures. The clastic rocks are not ideal 

 cement raw materials; the few analyses which have been per- 

 formed show that the composition is not uniform, silica varying 

 from 58 to 60 per cent, iron oxide from 5 to 12 per cent, and 

 alumina from 13 to 19 per cent. Their physical character is 

 likewise objectionable because of the coarse and nonuniform 

 grain size and the incipient schistosity. They are available in 

 enormous quantity, however, and in as much as the coast lime- 

 stones require so little clay that any change of composition in 

 the clay would change the composition of the entire cement 

 mixture to a much lesser degree they were considered as 

 possible siliceous materials. 



Siliceous material 51. — The shale (sample 51) which pre- 

 dominates in the coal-bearing rocks is a bedded deposit of 

 uniform physical character and very fine grain. The most con- 

 venient quarry site in this material is at Buntun on Pandan 

 River about 9 kilometers from the coast. More than an adequate 

 quantity is available, and could be quarried cheaply since the 

 shale is relatively soft. As with the clastic rocks, however, the 

 chemical composition of this material appears to be variable. 

 In view of the rather high content of silica in the coast limestones, 

 the shale might prove useful on account of the low ratio of silica 

 to alumina plus iron oxide (silica, 42 per cent; alumina plus 

 iron oxide, 18 per cent). 



CONCLUSIONS 



The coast limestones, represented by samples 35, 37, and 39, 

 are preferable to any other available calcareous material by 

 reason of their favorable situation, their softness, the nearly 

 horizontal position of the beds, the absence of overburden, and 

 their close approach to the desired raw mixture in chemical 

 composition. The chalk, which has the composition of sample 

 37 or 39, is preferable to the fragmental coral with the average 

 composition of 35, 37, and 39, because of its greater uniformity 



