28 GEOLOGY OF ASPEN MINING DISTRICT, COLORADO. 
are cited by Mr. Diller. Other occurrences of sandstone veins or dikes 
have been described by various writers, many of them referring their origin 
to sedimentation from aboye, in open fissures. Mr. Cross’! has described 
some interesting sandstone dikes in granite in the vicinity of Pikes Peak, 
but does not offer any suggestion as to their origm. The sandstone veins 
at Aspen resemble some of the grits of the Maroon series, which lie several 
hundred feet above the Leadville dolomite, and are separated from it by the 
Weber shales and limestones, as well as the blue Leadville limestone; on 
the other hand, there underlie this dolomite the various sandstones, quartz- 
ites, and dolomites which have been described. While, therefore, there is 
not sufficient evidence for any definite proof, the conditions are equally 
favorable for the application of Mr. Diller’s theory and for the theory that 
they were filled by waters which penetrated downward. 
Worm tracks in dolomite —A peculiar phenomenon, which excites much curious 
interest among the miners, is the occurrence in mine workings of fragments 
of hard blue dolomite riddled by small cylindrical cavities, which are at 
once recognized as wormholes. Such specimens are found in the Free 
Silver shaft. 
In specimens which do not show this perforation a similar structure is 
recognized under the microscope. In cross section there appear irregularly 
rounded or curved, generally elongated areas, which are perfectly distinet 
from the main rock, as if cut out. These are filled with crystalline dolo- 
mite, coarser in texture than the rest of the rock. In every way these areas 
seem identical with the worm tracks which are not uncommon among the 
fossils of limestones and other rocks. They were evidently made at an early 
stage in the history of the rock, when it was still plastic, and the cavities 
thus produced were filled with vein calcite or dolomite. Where, as im the 
Free Silver shaft, the cavities are now found empty, it is probable that the 
filling has been dissolved out by circulating waters. On the walls of these 
holes there are often very small crystals of pyrite. 
THE BLUE LIMESTONE. 
Description —I'he limestone which overlies the Leadville dolomite is blue- 
gray in color, sometimes dark blue; it occurs in massive beds, and the 
outcrop usually weathers light blue, with a smooth surface. The difference 
1Bull. Geol. Soc. Am., Vol. V, p. 225. 
