PETROGRAPHIC CHARACTERS. 13 
While there has been substantial agreement as to the general microscopic and 
chemical character of roofing slates, there have been questions as to the origin of 
some of their constituents. Is the muscovite (sericite) which makes up from a third 
to a half of the mica-slates the product of the metamorphism of argillaceous material, 
or are these shred-like scales the result of the disintegration of some micaceous rock, 
the effect of pressure having been simply to bring the shreds into parallelism and to 
mat them together? A shale under the microscope shows a considerable amount of 
apparantly clastic muscovite in scales of various sizes with more or less arrangement 
in the direction of the bedding. Sorby regarded the micaceous mineral of slates as 
formed in situ by an alteration of partially decomposed feldspar, yet admitted that 
the structure was just such as would result from the deposition of material sorted by 
gentle currents and subsequently compressed, but the chlorite he considered as un- 
doubtedly secondary. « 
Rosen busch wrote: 
Of course the material of clay slates was mechanically brought together, but the mineral constitu- 
ent of that part which is mainly micaceous and without feldspar was certainly the result of metamor- 
phic processes which were intimately connected with dynamic-geologic processes, ft 
Hutchins expressed his conclusions thus: 
This fine mixture of biotite, muscovite, kaolin, the minutest waste of feldspar, and in less degree 
of quartz, and probably other substances, under the joint action of pressure, warmth, and mineral 
solutions, gives rise to various decompositions and recombinations which result, among other things, 
in the formation of new mica, with the separation of titanic acid in the form of rutile. Into these 
reactions, whatever may be their exact course, even the muscovite in very fine state of division appears 
to enter: and there is good reason to conclude that in fine-grained sediments of suitable composition, 
exposed long enough to the necessary conditions as to pressure, temperature, and percolation of solu- 
tions, an almost complete regeneration of the " paste " to mica can and does take place, and that I his 
regenerated material, under in tenser dynamo-metamorphic action, is converted into some of the forms 
of micaceous slates known to us. The mica so formed is probably what in its more advanced stages 
of development is often known as sericite. c 
As slates are evidently derived from the waste of granitic rocks this material must 
have contained clastic mica, as do the shales, and wherever the feldspar of the granite 
or gneiss had previously been sericitized such sericite must have found its way into the 
sediments and into the slate without very great loss by decomposition. As a matter 
of observation, line-grained mica-slates do contain occasional scales of muscovite apart 
from the matrix, which may be of clastic origin, but the micaceous matrix of mica- 
slate is regarded as mainly of metamorphic origin. 
A question has also been raised as to the origin of the rutile needles. Roth^ is 
decidedly of the opinion that they belong to the original sediment. Thi'irach, e Pfaff,/ 
and Crednertf find an abundance of them in clay. Rosenbusch h states that while 
zircon and apatite bear traces of their clastic origin, such traces are entirely absent 
in both the rutile and the tourmaline. It will be noticed in looking over the 40 micro- 
scopic analyses of rooting slates made for this volume by the writer that the abundance 
of these needles varies greatly. In some very crystalline slates (Peach Bottom, 
Maine) they are hard to find, and in some less crystalline ones (Arkansas, Vermont) 
they are very abundant, 
The sizes of the mineral particles and the numerical abundance of some of them, as 
determined during the microscopic study of the roofing slates of Arkansas, California, 
a See On the structure and origin of noncalcareous stratified rocks: Quart. Jour. Geol. Soc. London, 
vol. 36, pp. 67-77. 
bNeues Jahrb. fur Min., etc., 1881, vol. 1, p. 399. 
c Hutchins, W. M., Clays, shales, and slates: Geol. Mag., vol. 7, p. 317. 
d Doubtless some of the constituents are of secondary origin (as quartz, pyrite, and the products of 
its oxidation, calcite, hematite, and limonite, gypsum), but the evidence of the secondary origin of 
the rutile needles does not seem to me convincing. Unstratified sands also contain a series of min- 
erals.— Allgem. u. Chem. Geologie, vol. 2, p. 586. 
e Thi'irach, H. , Ueber das Vorkommen mi kroskopischer Zirkone u. Titan Mineralien in den Gesteinen : 
Verh. phys.-math. Gesellsch., Wurzburg, N. F., vol. 18, No. 10, 1884. 
/ Pfaff. E., Petrographische Untersuchungen fiber die eocenen Thonschiefer der Glarner Alpen: Sitz- 
ungsb. math.-phys. Classe k. k. Akad. der Wissenschaft., Munich, vol. 10, 1880, p. 479. 
&Credner, G. R., Die Krvstallinischen Gemengtheile gewisser Schieferthone und Thone: Zeitschr. 
ges. Naturwiss., Halle, 1874. 
^Elemente der Gesteinslehre, p. 424. 
