ECONOMIC GEOLOGY OF THE BAGUIO DISTRICT. 483 



consists of a- fin^^grained, loose deposit of silic?i, nothing • more or lessr 

 than silicious sinter. At others it is an exceedingly hard chert, at still 

 other points it can be seen that it is plainly an altered andesite, or again, 

 a metamorphosed limestone. 



-At the time when the fissures in this country were being filled with 

 vein matter hy ascending silicious waters, many springs existed through- 

 out the region, in fact there are some left at wide intervals. It was 

 at this period, it is reasonable to suppose, that the various formations, 

 now found in an altered condition, were permeated by these waters and 

 many of the original constituents replaced by silica. 



;Thin sections of samples taken from this formation have been pre- 

 pared and 'examined with the microscope and show most conclusively 

 Just what has. taken place. One specimen, which looks like chert when 

 seen with the- naked eye, shows when examined with a high power 

 petrographic microscope, that the rock was originally a porphyritic an- 

 desite. The outlines are intact but the interior of the crystals is filled 

 with secondary cr)^ptocr3^talline quartz (Plate I). 



A place occurs on the "Military Cut-off" road just south of Govern- 

 ment Center where formerly one of these hot silicious springs was 

 located. The material is almost snow-white, very fine, and can be 

 scooped out as if it were loose sand. Some of this sinter has been as- 

 sayed and found to carry from 80 cents to 1.40 dollars in gold values. 

 An analysis of this material shows that it is made up of about 94 per 

 cent of silica.^ 



The later sedimentaries. — These have already been described by 

 PJveland in the article referred to. Further mention of the limestone will 

 be made in the chapter dealing vdth economic materials. ' 



GEOLOGIC HISTOBY, 



The Baguio region was comparatively high ground when that about 

 Manila and the Visayan islands was under water. The rock consisted of 

 diorite and perhajos old sediments, the age of which we do not know. 

 This land-mass was eroded, the sediments being deposited in the sea 

 around it. 



A volcanic period set in during which andesite flows spread over the 

 country, covering the sediments where they overlapped the basal mass. 

 The region then sank until the basal igneous mass in part was about 

 at, or perhaps below, sea level. A great reef-building period then 

 ensued and a considerable thickness of limestone was deposited. At the 

 close of the Miocene, the land began to rise. This was the geological 

 event known as the "Miocene Eevolution," which occurred throughout the 

 distance from Spain to the Pliilippines. At this period the gi'eat chains 



. .-^lAnalysis by 'A.- P. West, laboratoi-y of physical and inorganic chemistrjf, 

 Eurean of .Science. ... 



