TRIASSIC COAL AND COKE OF SONORA, MEXICO 13 



of sandy slate that its presence would be unsuspected until it thickens suddenly 

 into a laccolite of 20, 30, or more feet, throwing the overlying beds entirely out of 

 their regular course. The greater part of the flexures of the area are due to such 

 laccolites. 



The largest one of these laccolites is an intrusion of diorite just west of Tara- 

 humari. It has a length from north to south of nearly half a mile and its breadth 

 is but little less. Its thickness is more than 100 feet. It lies between the Tara- 

 humari sand and the underlying slate, and in places has been forced between the 

 slates as well, until it now appears to be interbedded with them. 



Some of the exposures of trachytic rocks look very much like sandstones, and 

 the resemblance is heightened by the pebbles of flint (?) and of graphite, which 

 they often carry in considerable numbers. 



While the intrusive rock probably passes from one bed of slate to another, only 

 one or two such breaks have been observed here. It is on account of this inter- 

 bedded condition that the coal beds of this area are workable, and to it is largely 

 due the presence of workable beds of coke. 



While some of the coal has a bituminous structure, analyses show that it is all 

 anthracite. It breaks with square, even fracture, has splendent luster, black 

 powder, and is not as hard as Pennsylvania anthracite. It contains from 4 to 8 

 per cent moisture and about the same percentage of ash. The volatile hydrocar- 

 bons are under 5 per cent, and the fixed carbon ranges from 76 to 85 per cent. 

 Such tests as have been made in burning it have shown very good results ; specific 

 gravity, 1.70 to 1.75. 



The coke is dark gray to grayish black in color, metallic to submetallic luster, 

 breaks with even fracture, and shows columnar structure like oven coke. Powder 

 black ; pores smaller and coke denser than most oven coke. In places it appears 

 lamellar, as if pressure had flattened the walls of the pores. It is a good fuel in 

 blacksmith forge, open fire, or assay furnace, burning steadily without deflagration, 

 and will probably be entirely satisfactory for all metallurgical uses, except such as 

 may require a greater porosity. 



The coke is simply a local condition of the coal, largely due to the presence of 

 the igneous rock in immediate proximity of the bed. 



In two of the principal openings on the coke the igneous rock practically forms 

 the roof, and in a third instance it forms the floor of the seam. In these openings 

 there are occasional inclusions of the igneous lock in the coke, and there are other 

 places where it thickens and cuts the coke out almost entirely. The plastic nature 

 of the igneous rock at the time of its intrusion is shown by the way in which the 

 coke is mixed through it in these horses, if they may be so called, and also by the 

 presence in the same intrusive rock, in other exposures, of fragments of graphite, 

 which represent the passage of the plastic material along or through some coal bed. 



In other beds, however, coke appears without any igneous rock near it, so far as 

 we can find. In one seam of anthracite, which has a thickness of 1 feet, we find 

 pockets of coke near the center, and in one coke bed we find pockets of anthracite 

 near the base. In two or three beds we find both coke and anthracite present, but 

 in different benches. In one case there is a clay parting 3 inches thick between 

 the coke and underlying anthracite, but in the others no parting is found, and in 

 one the anthracite is on top and coke below. 



In one coke bed there is some tendency to concentric structure, and kidneys or 

 eggs of coke are scattered through the more massive material. 



In the vicinity of San Xavier and Los Bronces, north and east of here, some 



