168 TT. M. Hutchirigs — An Interesting Contact-Hock. 



In addition to the particular minerals, it is not unlikely that some 

 of the "spots" so frequent in altered slates, etc., may be phenomena 

 which are special to contact-action, and not produced in any other 

 way, even when we are not able to trace them directly to intrusions. 



Zirkel, in his recently completed Petrographie (vol. iii. 1894), in 

 dealing with the mica-schists, phyllites, etc., alludes at several places 

 to the fact that spotted rocks occur among them, where we have 

 no direct evidence of contact-action. In commenting on this, at one 

 place, he speaks cautiously as to there being no " recognizable 

 connection with eruptive rocks " ; at another place he speaks more 

 dogmatically as to the spotted schists being in no way due to 

 contact-action. As we know that spots are produced at considerable 

 distances from the actual contacts, and as, apparentlj^ characteristic 

 rocks of this nature have not been shown to be produced in any 

 •way except by intrusions, it is surely not safe to adopt any decided 

 view, based simply on the absence of visible igneous rocks. 



One of the most interesting occurrences of spots known to me is 

 in some of the rocks of the Ardennes, which are not, I think, 

 regarded as contact-rocks, but which are always, on the contrary, 

 described as standard examples of " regional " action. These spots 

 do not seem to have been noticed by Eenard and others in their 

 examinations of these rocks, 



I have a specimen of the well-known "coticule" of Viole Salm, 

 with part of the contiguous band of dark slate attached to it, for 

 which I am indebted to Prof. Eenard, The cotioule is light 

 yellowish in colour. Very thin sections show it to be made up 

 mainly of white mica, mostly lying flat in the plane of bedding, 

 with the abundant manganese-garnets and rutiles disseminated very 

 evenly all through it. There is a little quartz; and a total absence 

 in the slides of any darker minerals or pigment. Viewed in ordinary 

 light, the whole field is quite uniform, but in polarized light large 

 numbers of dark spots appear. They are seen to be framed more or 

 less strongly in rims of extra-accumulated white mica, just as spots 

 are often framed in dark mica and so rendered conspicuous in 

 ordinary light. These rims of colourless mica do not show at all 

 in ordinary light, and one is unable in any way to make these spots 

 stand out from their surroundings, by adjustment of illumination or 

 otherwise, the gai-nets and rutiles being present in them in the 

 same amount and uniformity of dissemination as in the rest of 

 the rock. With crossed nicols they are seen to be isotropic, but 

 to contain, in addition to the garnet and rutile, lai-ge numbers of 

 dimly-polarizing, very minute grains and flakelets which cannot be 

 determined. The average diameter of these spots is about too- inch, 

 and sections cut parallel and vertical to bedding show that they 

 are approximately equal in all directions, a large proportion being 

 spherical or oval. The study of sections in various directions also 

 disposes of any idea that there is here any compensation in question; 

 the main material, the groundmass, of these spots is really isotropic 

 and colourless. There are so many of them that in a section 

 measuring i inch by -1% inch, over one hundred can be counted^ 



