218 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
and the chert seams follow the fractures the plane of jointing may easily 
be mistaken for that of stratification. Under the microscope the process of 
formation of this chert is seen to be an introduction of many tiny quartz 
erains along slight areas of shearing or fracturing, such areas being 
usually marked by microscopic zones of brecciation. When this process 
has gone on sufficiently far a chert band results. This chert is made up of 
cryptocrystallme or chalcedonie silica, and usually contains many small 
bodies of carbonate. Some of the larger bodies are irregular and evidently 
residual, while the smaller ones are mostly concentrated into rhombohedra, 
which commonly show a zonal structure, indicating growth by successive 
additions. 
It is m the Leadville blue limestone, however, that the process of 
silicification along fracture zones is best shown. The various stages 
of change by which a rock consisting almost entirely of carbonate of lime 
is transformed mto one made up chiefly of quartz have been carefully 
observed under the microscope. Often in a single section a change from 
typical foraminiferal limestone to quartz is illustrated in all stages, one part 
of the section contaiming only quartz, while another shows no change from 
its original condition. Usually, however, the process takes place grad- 
ually, and its beginning is signalized by the appearance in the limestone of 
many small, irregular quartz grains. Scattered here and there is also 
encountered a long and slender crystal of quartz which lies entirely sur- 
rounded by the fresh limestone, like a porphyritic crystal in an igneous 
rock. As the process of silicification progresses these slender crystals 
multiply, so that they finally jom and form a peculiar and characteristic 
netlike or retiform structure. This network of imperfect quartz prisms 
incloses areas of calcite which are sprinkled with small, irregular quartz 
grains, varying in size down to the very smallest dimensions. Many of 
these small grains may be cross sections of prisms; they have not, 
however, hexagonal outlines, and their forms are irregular and often 
angular, recalling the universally distributed tiny quartz grains in the 
bedded dolomites. The final result of this silicification is the formation of 
a rock made up almost entirely of crystalline quartz grains of various 
sizes and shapes, in which the retiform structure is still predominant. 
There are also occasional small areas of crystalline calcite, which are 
probably residual. Accompanying the silicification is almost invariably a 
