182 



ON THE TOPOG-liAPIIY A'NT) CIEOLOGY 



while at its western base, at ]^ovillero, it is brownish, much cracked, and with the 

 surfaces stained with oxide of iron. Further south, at Arbol Gordo, it is sometimes 

 a Httle magnesian and is usually much more slatey. A little further south still, in 

 the woods of Monte Pueblo and on Madrigal Creek, it varies from a clay to a talcose 

 slate covered with a heavy red soil, the rock traversed by innumerable little quartz 

 veins rich in gold. The whole surface of Monte Pueblo is auriferous — the greater 

 part if not all the soil would " pay " for washing — but unfortunately the deposit is 

 too shallow to warrant the expensive ditching necessary to carry water from the 

 Jaina to a height of perhaps forty feet above its level, opposite that point, to reach 

 the required level The aggregate quantity of gold is probably not sufhcient to cover 

 the expense of a ditch of two or three miles long that would be necessary. I have 

 obtained from an average of a dozen to a maximum of forty " colors " or specks of 

 gold from a single panful of dirt over the greater part of this area. Up the Madrigal 

 Creek, on the eastern margin of this tract, I found jaspers, clay-slates, sandstone, and 

 a peculiar serpentinoid rock in place. The latter, a dark gray, contained little con- 

 cretionary grains scattered through it of a lighter color and a little harder ; so that 

 weathered surfaces took on an appearance, except in color, similar to that presented 

 by mica slate studded with garnets. The rocks on the west side of the Jaina, above 

 the mouth of Madrigal Creek, differ but little from those already described, the 

 most common form being claystones varying from brown or gray to nearly black. 



At the mouth of the Madrigal, on the southern edge of the Monte Pueblo or 

 " Buenaventura " tract, the slates are all highly jaspery and are here penetrated in 

 every direction by syenitic dykes of all sizes, often less than an inch in width and 

 occasionally many feet. The syenite here is composed of nearly equal parts of the three 

 usual constituents and is of a light gray color, sometimes containing little masses of 

 the same material of a finer grain, and not rarely pieces of the enclosing jasper. It 

 is perfectly cemented at its walls, so that no amount of force or blows will separate 

 the two rocks along the line of juncture ; fractures ci'ossing from one material to the 

 other perfectly. I collected numerous hand specimens here and elsewhei'e, in part 

 green or blackish jaspery slate, the remainder syenite. 



While at Madrigal I availed myself of the rich assortment of boulders in the bed 

 of the river to collect a characteristic sei'ies of those syenites in which the hornblende 

 predominated. This form of the rock seems to be peculiar to the Upper Jaina. I 

 never saw it in place, but from the fact that it forms perhaps ten per cent, of all the 

 pebbles in the river, it is not probable that it owes its origin to a single dyke. In 

 some of the specimens the hornblende exists as rather isolated crystals sparsely 



