94 
PALAEONTOLOGY OF NEW-YORK. 
strata showing infinite gradations between the slate and sandstone; intermixtures 
and interlaminations of the one and the other, and all possible modifications of 
these two simple materials in the sedimentary deposits : the admixture of calca¬ 
reous matter producing calcareous shale and calcareous sandstone, and giving a 
less or more calcareous character to all the intermediate varieties of these rocks, 
and finally the development of limestone. All these are familiar to him; and as 
he approaches the changed forms of these rocks, and sees the beginning of meta¬ 
morphism, and the gradual development of the segregated and crystallized mine¬ 
rals, he still looks upon these rocks in the mass as strata of shale, sandstone, and 
the intermediate varieties of rock made by the mingling of these and the accession 
of calcareous matter. The fundamental rock still holds its place in the mind of the 
geologist, and the gradually assumed crystalline structure and the segregation of 
crystallized minerals does not make him forget the normal condition of the mass ; 
and he knows that the occurrence of these minerals is due to chemical action, and 
that the original character of the beds has determined the kind of mineral which 
shall be developed when the mass is subjected to this influence, or placed in a 
condition to allow of free molecular motion. The metamorphism has only given 
combination and form to materials before existing in the rock : it has produced 
nothing new. 
Since, in the unaltered condition of these strata, no two shales or sandstones 
or any intermediate gradations of these rocks, nor any two limestones in the se¬ 
quence, preserve precisely the same characters ; so is it impossible that we should 
have, in the metamorphic strata, any two sets of beds of precisely the same cha¬ 
racter or giving origin to the same minerals, with the same modifications of form, 
precise composition, etc. The difficulty of characterizing the metamorphic rocks 
by strict terms without qualification, is equally as great as among the unaltered 
rocks in designating all the gradations from shale or slate to sandstone, and all the 
modifications produced by a greater or less admixture of calcareous matter with 
the ever varying proportions of the other two. 
In the western flank of the Green mountain range, the great variety of schists 
designated as talcous, mica, gneissoid mica, hornblende and calcareous mica slates 
are all results of the metamorphism of Silurian strata, and, to a great extent, 
express the gradations which, a little farther west, are represented in the Utica 
slate, the Frankfort slate, the Pulaski shales and sandstones, and the upper 
sandstones and conglomerates of the Hudson-river group, together with the 
Medina sandstone and the strata of the Clinton group and associated calcareous 
beds, including also some pari of the Niagara group. At the same time it has been 
clearly shown, from investigations in the Canada Geological Survey, that the inter¬ 
calated, irregular, and interrupted beds of magnesian limestone in the upper part 
of the Hudson-river group become the serpentines of this mountain range. 
