210 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
within 6 miles of the imaginary converging point, while the trend of 
all the dikes sufficiently known to permit a statement concerning their 
directions is essentially at right angles to these fractures, being north- 
west instead of northeast. 
A well-defined northwest system of fractures, to which probably all 
the known dikes of the district belong, finds its best expression in the 
western half of Crittenden County. Here the trend of tin' dikes and 
faults belonging to the system varies between N. 30° W. and N. 37° W. 
The fractures of this system usually caused only a very limited dis- 
placement, but they contain some of the largest mineral deposits of 
the district, notably at the Eureka, Old Jim, and Holly mines. 
The northeast faults found in the northern and eastern parts of 
Livingston seem to indicate a distinct third system, extending across 
the Ohio from Hardin and Pope counties, 111. Similarly, the north- 
west fractures occurring in the northern parts of Crittenden and Liv- 
ingston counties, having a direction varying but a few degrees either 
way from N. 20° W\, probably belong to a fourth system, which, Like 
the other, has its strongest development in the Illinois counties 
mentioned. 
The fractures, whether mineralized or not, frequently furnish chan- 
nels for descending underground waters, as is evidenced by the cor- 
rosion of the walls, forming in the case of some of the apparently 
unmineralized fractures open fissures or crevices filled with nn] clay. 
Sink holes are common along some of the fractures, and caverns are 
known to follow them for short distances. 
The formation of the mica-peridot ite dikes, of which seven or eight 
are known in Crittenden County, and one in Pope County, 111., is 
believed to have taken place prior to the extensive faulting of the 
region. They were probably produced by an accumulation of molten 
matter within this portion of the crust of the earth, causing its eleva- 
tion and fracturing and subsequent intrusion of the igneous masses. 
The strain on the continuity of the strata produced by 1 heir elevation 
caused the relatively brittle limestone to part along certain lines and 
form fissures. The more i>liable shales and sandstones of the Chester, 
however, frequently accommodated themselves to the strain, so that 
the intruded mass failed to pass through them, but spread itself hori- 
zontally in sheets between the bedding planes. The fissures occupied 
by the dikes are generally very nearly vertical and (pi ite straight in 
their courses, and although narrow, varying from about 2 feet to 
nearly 25 feet in width, some of them have been traced for miles. 
THE VEINS AND VEIN MINERALS. 
By W. S. Tangier Smith. 
The well-defined veins of this district almost without exception fill 
fissures due to faulting. They are found in the Princeton, St. Louis, 
and Chester formations; mainly in the first two. Where two of the 
formations have been faulted into juxtaposition, veins frequently 
