74 W. M. Hutchings — Clays, Slates, and Confact-MetamorpMsm. 



work of Vernadsky, in wliicli it was shown that when cyanite is 

 heated to 1300°— 1400° C. it is altered to sillimanite. 



Mr. Barrow has made it appear very probable that in the hotter 

 zones of contact sillimanite will be formed, that cyanite will take its 

 place when a certain limit of lower temperature is reached, and 

 staurolite will occur in the still cooler outer zone. No doubt these 

 observations of Mr. Barrow in this particular district will cause the 

 point to be carefully studied elsewhere. 



Mr. Barrow further infers from Vernadsky's experiment that we 

 may conclude that in the " sillimanite zone " of contact-rocks there 

 has been a temperature of 1300° — 1400° C. or over. This seems 

 rather straining the matter ; and certainly attempts in the past to 

 draw conclusions as to rock-temperatures fi'oni experiments on 

 minerals in the laboratory have not been so happy in their results 

 as to encourage repetition, however tempting a system of " recording 

 pyrometers " in the rocks may appear. It does not seem in the 

 least necessarily to follow from Vernadsky's experiment, either that 

 the sillimanite in the rocks was produced from cyanite, or that the 

 formation of sillimanite instead of cyanite in the hotter zones indicates 

 that the temperature was 1300° — 1400° G. or that there need be any 

 definite relationship whatever between the temperature at which it 

 forms in these rocks and that at which it may be produced, from 

 another mineral, in a laboratory-furnace. 



If contact-metamorphism be due entirely to the thermal effects of 

 the intrusive rock, as now seems usually to be believed, we should 

 expect to see these effects always the same, in kind, on the same 

 variety of rocks undergoing metamorphism, and the alterations due 

 to a dolerite should not differ from those due to a granite. In the 

 main this seems to be borne out, but there are some important 

 points which still call for a good deal of attention. 



I may state that so far as my own observations go, in which I am 

 still engaged, there is, for instance, a formation of the same sub- 

 stance, due to the same causes, as I have described above ; but it is 

 present in less amount, and a larger proportion of it remains in the 

 amorphous, or slightly developed, state. Also there is much less 

 formation of " mosaics " and these again much less developed. 

 These facts can be easily understood by having regard to the 

 relative smallness of the masses of igneous rock acting in the 

 observed occurrences, and the consequent less time during which 

 recrystallization would be able to go on. 



It does not appear that isotropic substance is much mentioned 

 by observers of altei'ed slates, etc., at gframie-contacts ; but it is 

 frequently spoken of where the intrusive rock has been " diabase." 



The most striking difference, however, between acid and basic 

 contacts, so to speak, lies in the fact so often recorded, and backed 

 up by so much evidence, microscopic and chemical, that there is a 

 transfer of material from the intruding basic rock to the slates, etc. ; 

 a transfer which is large in proportion to the relatively small masses 

 of igneous rock concerned in it. It is shown by the increase of 

 soda in the altered rocks, chemically considered, this increase being 



