VOLCANIC TUFF. 393 



In addition to the rocl^s mentioned above, volcanic sediments and 

 pyroclastic tnfEs occur quite widely distributed in the Philippines. They, 

 are especially abundant in west central Luzon, extending almost un- 

 brokenly from near Lmgayen Gulf to the seacoast of Batangas, practically 

 blanketing or covering nearly all of the massive rocks of this region. 

 Mr. Ickis found volcanic tuff in the Agusan-Pulangui region, interior 

 from Cagayan, Misamis. 



In the absence of a better stone, in certain places this has been used 

 extensively for building purposes. In Bulacan and along the Pasig 

 Biver, especially near Guadalupe, this stone is unusually abundant. 

 Large quantities of it have been quarried and used in the construction 

 of many churches and other buildings and in the walls and fortresses of 

 Manila. It is very workable. Before it is disturbed it is so soft that it 

 can be quarried with an axe, but it hardens rapidly on exposure. 



Since American occupation the competition of brick and other manu- 

 factured products in Manila has encroached upon the general use of 

 volcanic tuff, nevertheless a large quantity still finds a market. Owing 

 to the extended use of this stone I have made a few experiments to show 

 its real value. The most important laboratory tests which aid one to 

 form an opinion as to the value of a stone are the microscopical, physical, 

 and chemical examinations. Such tests are given below : 



MICKOSCOPICAL EXAMINATION. 



A microscopic examination of the tuff in the vicinity of Manila shows 

 it to be composed of (1) plagioclase, both decomposed and undecom- 

 posed. There seem to be two generations of feldspar; the one rounded 

 and largely decomposed, the other rather angular and in appearance as 

 if it had come from a greater distance, (2) magnetite, (3) hornblende, 

 (4) quartz grains, (5) the cementing material, which is probably in 

 greater part volcanic ash, is largelj' composed of oxide of iron. It might 

 also be mentioned that a certain amount of pumice is nearly always to 

 be found in this tuff. It is undoubtedly andesitic tuff. Silica is the 

 strongest and most desirable bonding material because it is insoluble, 

 free from cleavage and has the same coefficient of expansion as the sand 

 grains which it holds together; however, iron oxides make a fair bond. 

 The latter are more desirable as a cementing material than calcite which 

 is soluble, has very pronounced cleavage, and has a different coefficient 

 of expansion from that of the mineral grains which it holds together, or 

 than clayey matter which is liable to disintegrate. 



PHYSICAL EXAMINATION. 



The crushing strength of a few samples of tuff has been determined. 

 The object of such tests in stone is to discover the relation between the 

 crushing strength and the stress to wliich it would be subjected in a high 



