W. M. Hutchings — An Interesting Contact-RocJc. 123 



position may differ a good deal quantitatively ; and it is partly to 

 such initial variations, and partly to consequent differences in the 

 inetainorphic action of the igneous rock, that we must ascribe the 

 differences that we find in the microscopic examination of slides 

 from several points in this bed, which is macroscopically a fairly 

 uniform mass. 



To give an idea of this interesting rock it will be best to describe 

 separately two or three typical sections, whose nature will give 

 a good general idea of the various points to be observed at this 

 exposure of the bed in question. 



Thus, from numerous hand-specimens collected near together, several 

 slides have been made in which some of the spots have been 

 included ; in this case spots which are firmly incorporated in the 

 rock and cannot be detached. 



In ordinary light a very thin section shows the main portion 

 of the rock to be composed of a uniform gray mass, in which are 

 bedded abundant grains of clastic quartz of an average size of aljout 

 -aV inch, but ranging up to as much as iro inch in diameter. The 

 gray ground-mass shows a vast number of rutile-crystals of very 

 small size. They are not in the form of the " clay-slate needles," 

 but in the relatively short and blunt form, with definite terminal 

 developments, so usual in altered rocks. There are also the 

 numerous small zircons, and the small hemihedral crystals of tour- 

 maline, so characteristic of clays, shales, and slates. In all respects 

 this rock corresponds to an altered moderately quartzy clay or shale, 

 such as can be examined in any number of instances in the Carboni- 

 ferous beds 



In poLirized light this gray groMnd-mass shows, however, most 

 intei'esting developments. Most of it is seen to consist of rounded, 

 irregular, or more or less definitely bounded grains of a mineral, 

 or minerals, lying bedded in and surrounded by a material which is 

 mostly quite isotropic, but at other times shows an indistinct effect 

 of birefractive substances developing within it. 



At some places we have the polarizing grains closely packed 

 together, with so little of the surrounding isotropic material that 

 they amount to a true interlocking mosaic; at others they are very 

 much more separated, and we have quite large spaces of the sur- 

 rounding material, perfectly isotropic, but containing the swarms 

 of minute rutiles, small flakes of white mica, and a certain amount 

 of indistinct dusty and granular substances diffused in it. 



A large portion of the polarizing grains can be proved to be 

 quartz, some of them can be identified as felspar, though this 

 is more difficult, and a good proportion cannot be made to give any 

 definite optic tests at all. The grains are not yet water-clear, nor 

 yet so sharply defined as those of a perfectly developed contact- 

 mosaic ; there is a certain amount of dimness in them, which is 

 accentuated also by their envelopment in isotropic base full of 

 microlites. It is, however, certain that we have here a production, 

 due to the metainorphism of the shale, of quartz with more or less 

 of felspar. All these grains are in evei-y way perfectly and sharply 



