. On the microscopic structure of Stromatoporidae [microform] : and on paleozoic fossils mineralized with silicates in illustration of Eozoon. Paleontology; Paléontologie. w 60 J. W. DAWSON ON PALEOZOIC FOSSILS. or shales, with thin bands of limestone holding fragments of Lower Silurian corals and crinoids. These pass upwards into a thick series of slaty rocks characterized by the prevalence of a shining crystal- lino hydro-mica, and known as nacreous or hydro-mica slates. They are associated with quartzose bands, and also with lenticular layers of crinoidal limestone. Parallel with these beds
. On the microscopic structure of Stromatoporidae [microform] : and on paleozoic fossils mineralized with silicates in illustration of Eozoon. Paleontology; Paléontologie. w 60 J. W. DAWSON ON PALEOZOIC FOSSILS. or shales, with thin bands of limestone holding fragments of Lower Silurian corals and crinoids. These pass upwards into a thick series of slaty rocks characterized by the prevalence of a shining crystal- lino hydro-mica, and known as nacreous or hydro-mica slates. They are associated with quartzose bands, and also with lenticular layers of crinoidal limestone. Parallel with these beds and, according to Logan's observations, overlying them, is the series containing the serpentine, which is associated with layers of limestone and nacre- ous slate, and also with brecciated and arenaceous beds, probably originally tufacoous, with beds of auorthito, steatite, and dolomite, and also with red slates, the whole forming a miscellaneous and ir- regular group, evidently resulting from the contemporaneous action of igneous and aqueous agencies, and affording few traces of fossils. The serpentines, which occur in thick and irregular beds, are differ- ent in colour and microscopic texture from those of the Laurentian system, and also present some chemical differences, more especially in the presence of oxides of nickel, chromium, and cobalt, and of a larger percentage of iron and a smaller proportion of water*. These serpentines are undoubtedly bedded rocks and not eruptive; but they may have originated from the alteration of volcanic mate- rials f. They appear, shortly after their original deposition, to have been broken up, so as in many places to present a brecciated ap- pearance, the interstices of the fragments being filled with limestone and dolomite, which themselves are largely mixed with the flocculent scrpentinous matter, and traversed by serpentinous veins some- times compact and sometimes fibrous. Besides the very impure limestone thus occurring in the serpe
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