> ° = 

eee 2 GEOGNOSY: . 3] Bdaaiae 
the remains of living organisms, either by growth on the spot or by 
transport and accumulation as mechanical sediment. To by far the 
larger part of the limestones intercalated in the rocky framework of 
our continents an organic origin may with probability be assigned. 
It is true, as has been above mentioned, that limestone, formed of 
the remains of animals or plants, is liable to an internal crystalline 
rearrangement, the effect of which is to obliterate the organic 
structure. Hence in many of the older limestones no trace of any 
fossils can be detected, and yet these rocks were almost certainly 
formed of organic remains. An attentive microscopic study of 
organic calcareous structures and of the mode of their replacement 
by crystalline calcite, affords, however, indications of former organ- 
isms, even in the midst of thoroughly crystalline materials.’ 
Limestone, composed of the remains of calcareous organisms, 
is found in layers which range from mere thin lamine up to massive 
beds, several feet or even yards in thickness. In some instances, 
such as that of the Carboniferous or Mountain limestone of England 
and Ireland, and that of the Coal-measures in Wyoming and Utah, 
it occurs in continuous superposed beds to a united thickness of 
several thousand feet, and extends for hundreds of square miles, 
forming the rock out of which picturesque gorges, hills, and tablelands ~ 
have been excavated. 
Limestones of organic origin present every gradation of texture 
and structure, from mere soft calcareous mud or earth, evidently 
composed of entire or crumbled organisms up to solid compact 
crystalline rock, in which indications of an organic source can hardly - 
be perceived. Mr. Sorby, in the address already cited, calls renewed 
attention to the importance of the form in which carbonate of lime 
is built up into animal structures. Quoting the opinion of Rose - 
expressed in 1858, that the diversity in the state of preservation of 
different shells might be due to the fact that some of them had 
their lime as calcite, others as aragonite, he shows that this opinion 
is amply supported by microscopic examination. Even in the shells 
of a recent raised beach he observed that the inner aragonite layer 
of the common mussel had been completely removed, though the 
outer layer of calcite was well preserved. In some shelly limestones 
containing casts, the aragonite shells have alone disappeared, and 
where these still remain represented by a caleareous layer, this has 
no longer the original structure, but is more or less coarsely 
crystalline, being in fact a pseudomorph of calcite after aragonite 
and quite unlike contiguous calcite shells, which retain their original 
microscopical and optical chavracters.? 
The following list comprises some of the more distinctive and 
important forms of organically derived limestones. jes 
Shell-Marl—a soft, white, earthy, or crumbling deposit formed 
' Sorby, Address to Geol, Society, February, 1879. 
2 The student will find the address from which these citations are made full o 
suggestive matter in regard to the origin and subsequent history of limestones. oO 

