86 
GEOLOGY OF THE FOURTH DISTRICT. 
24. 
This portion of the rock, as it appears in the vicinity of Rochester, is grey and subcrystal¬ 
line, very hard and tough, exceedingly irregular in its stratification, and difficult to quarry. 
Sometimes for a short distance the strata are regular, and then separated by seams of shale, 
and again all trace of stratification is lost, and the whole assumes the contorted and concre¬ 
tionary structure. It contains numerous cavities partially filled with crystalline materials, 
such as dogtooth and rhomb spar, gypsum, and more rarely sulphate of strontian and the sul- 
phurets of lead and zinc. In many instances, these cavities present a partially decomposed 
organic body, as a Favosite or some other coral, which seems to have formed the nucleus 
around which the stony matter accumulated. Sometimes the remains of these are distinctly 
visible ; at others, they are entirely obliterated, the form of the cavity being the only evidence 
of their having existed. 
25. 
In the accompanying illustration, which represents the remains of a Porites, the transverse 
laminae, indicating the stages of growth in the coral, are distinctly preserved, while all the 
intermediate parts are removed, only a few calcareous crystals remaining in the spaces. 
The succeeding portion of the rock, here as elsewhere, is of a grey or greyish brown color, 
often darker on first exposure, but always weathering to the grey shapeless masses, seen 
