W. M. Hatchings — Clays, Shales, and Slates. 345 



But we can find examples enough of contact-slates in which brown 

 mica and chloride mineral are developed together, and accompanying 

 a recrystallization of the micaceous mineral to muscovite ; or we 

 may have rocks in which a very high degree of recrystallization 

 has been reached without any brown mica being formed at all. 

 In such rocks we see, among the main mass of the slate-mica, the 

 crystallization of undoubted muscovite, at all angles to the plane 

 in which the slate-mica lies flat ; and we may see this in all degrees 

 up to the point at which we get a completely regenerated rock, 

 consisting mainly of a criss-cross mass of muscovite-flakes and 

 crystals. 1 



The newly-formed chloritic mineral is often seen to take a degree 

 of crystalline development and of individualization proportionate 

 to that taken by the mica ; where this latter mineral forms sharply- 

 bounded plates, they are not infrequently grown together with 

 equally sharply-defined individuals of chlorite, sometimes one of 

 each, sometimes one sandwiched between two of the other, and 

 clearly crystallized under the same conditions and at the same time. 

 We also see the chloritic matter sometimes take the form of 

 larger indefinite bodies, in which crystals of muscovite lie in all 

 directions, their cross-sections giving a structure exactly comparable 

 to the " ophitic " of igneous rocks ; or again, we see it jammed in 

 the angles among intercrossing mica crystals, in just the manner 

 which is called " intersertal " in the case of the ground-mass of 

 basalts, etc. ; all these structures distinctly showing the simultaneous 

 origin of the two minerals. 



In studying these recrystallizations due to contact -action, we 

 come again upon the question of the "spots" which are so frequently 

 found largely produced in just those less highly altered portions 

 of contact-areas which we are now considering. About two and 

 a half years ago I made some remarks and suggestions on these 

 little-understood spots (Geological Magazine, Jan. and Feb. 1894). 

 Since then I have paid a good deal of attention to the subject, 

 and examined a large amount of material, the result being, I 

 hope, to throw a little more light on their nature and mode of 

 origin, and to connect a large part of them with the processes of 

 rock-development which I have endeavoured to outline above. 

 In the paper in question I wished to show what good reasons 

 we have for a conviction that the striking recrystallizations we see 

 in contact-rocks are brought about by the means of some sort of 

 mineral solution which is formed in the sedimentary rock ; the 

 components of the original materials of the rock being more or 

 less taken up into this solution, and again crystallized out of it in 

 new forms of the same or other minerals. 2 



1 By crystals are here meant those individuals of muscovite which stand out in 

 distinct and definite plates, with perfectly parallel edges, sharply marked off from 

 any more confused mass of irregular flakes. 



2 For the consideration of the question here involved, it is quite immaterial 

 whether this solution is formed entirely by the action of heat and contained water on 

 some of the components of the sedimentary rock, or whether it has received additions 

 in the form of alkaline liquids from the intruded igneous mass. 



