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DIFFERENCES AND RELATIVE APPEARANCE OF IGNEOUS ROCKS. 



The further up the river is travelled, to Tres Alamos, the deeper this bed appears to become, 

 until the desert level at camp, (July 17,) at the crossing of the river, is reached. 



This gravel appears to be the covering of the whole plain country around, and, from its great 

 depth, is one of the causes of its great sterility. At the point where the river canons it is 

 about 100 feet deep. It always reposes uncomformably upon the conglomerate beds, which 

 are there exposed^ and which constitute the lower beds of the gypseous strata. 



At the base of the small hills northeast of camp at Tres Alamos, this conglomerate is exposed, 

 and, by pounding and washing the pebbles, quartz, red porphyry, and serpentine, a few small 

 scales of gold were obtained. Southwest of the river, at Calabasas ranch, and at Tucson, gold is 

 also obtained ; and there is no doubt that not only where this conglomerate is exposed, but 

 also in the granitic and felspar rock of Santa Oatarina, from whicb the conglomerate is derived, 

 gold could be obtained by washing. The scarcity of the water is the great drawback to its 

 abundant separation. The cementing clay of the auriferous gravel had very minute crystals of 

 carbonate of lime (arragonite) interspersed througbout. 



Of the two varieties of plutonic rock which appear along the Gila and in the San Pedro 

 valley — the basaltic and reddish amygdaloidal porphyry rock — the latter is a tracbhyte, and 

 exists in by far the largest masses, while the basalt occupies a stratum or layer a few feet thick 

 over the general surface ; the amgydaloid trachyte forms tbe great mass of basal rock in many 

 of the upheavals of this region ; yet it is remarkable that, upon the mesas and banks of the 

 San Pedro, the scattered fragments of basaltic rock which lie strewed over the whole surface are 

 very numerous, and constitute, indeed, the main portion of the detrital fragments ; the frag- 

 ments of amygdaloid rock bear no relation to those. 



On the other band, when we examine the conglomerates, especially those placed lowest in 

 position, the amygdaloid is found to be the prevailing rock,, giving the color and the character 

 to the conglomerate, then granitoid rock, and least of all the basalts ; in some beds the basalt 

 is wholly wanting. 



The basalt is found mostly in veins or dykes and the amygdaloid rock in masses, and, in 

 almost every instance, the latter is covered up by basalt. 



From the relative position of the two rocks, and from the constitution of the conglomerates, 

 it is evident that the basaltic flow is posterior, in point of time, to the elevation of the amygda- 

 loid rock. 



When reviewing the structure of the Organ mountains, it will be found that a similar view 

 of the relative ages of these plutonic rocks is advanced, based upon other reasons. 



The crossing at Tres Alamos, on the San Pedro, is over 900 feet higher than the mouth of 

 the same river 90 miles north. The valley has risen thus much toward the level of the hill tops, 

 and nothing but the desert gravel meets the eye roaming over one of the most extensive pros- 

 pects which nature can present — a flat of almost boundless extent toward the south, with the 

 horizon line in a curve from its vastness, broken in upon only at a few points by an isolated 

 range, the representation of the loftier and closely aggregated chains of New Mexico. 



