256 ©. Davies Sherborn—Concentric Structure in Limestone. 
there are neither of these evidences; but the last few regular rings— 
of a pale ash-grey colour——-are crowded with straight or slightly 
curved fibre-like markings, rarely bifurcating, sometimes solitary and 
sometimes in bundles. They are indicated by white linear streaks, 
and under the microscope are seen to be minute cracks through the 
layers. They do not radiate from the centre of the mass, and are 
apparently quite independent of the original crystallization ; for in 
some instances they are tangential to the series of rings. In some 
of the darker rings, radial crystallization is well marked, round the 
outer edge, and we can see distinctly that such layers would, if their 
Section of a concentric mass, probably of organic origin, imbedded in dark- 
coloured limestone, from Kulu, Central Himalayas. 
surface were exposed, exhibit a mammillated appearance. The 
fibre-like cracks and the radial crystallization are both too fine to 
show in the Figure, which represents the specimen about natural size ; 
but their position is indicated by the black triangles. The centre 
is a woolly-looking oval mass, three-quarters of an inch in its 
longest diameter, of a pale-grey colour, and may owe its appearance 
to its being semitranslucent and crystalline. 
We have neither granular nor oolitic material visible in the Indian 
specimen. Cracks run through it in several directions; but these 
