AlfNlVEESAET ADDEESS OE THE PEESIDENT. 9 1 



of the schists near Inverary and in Anglesey. The intermediate 

 kind, in which the stratification-foliation has been bent into small 

 contortions, and only a little mica has crystallized along the planes 

 of discontinuity, is met with more frequently. The differences in 

 the character of the foliation in the different specimens now de- 

 scribed appear to have depended on whether or not the rock became 

 completely solid and crystalline before it was compressed, or was 

 compressed before it became solid and crystalline. The intermediate 

 varieties were probably formed when there was a partial metamor- 

 phism before compression, and further crystallization afterwards. 



Schists not mechanically deposited as such. 



I have most carefully examined my various sections of schists, 

 in order to ascertain whether their structure would in any way 

 countenance the supposition that the constituent minerals were 

 originally formed as a loose crystalline deposit from the sea, and 

 not formed in situ by the alteration of a more normal deposit. If 

 such had been the case, we might very well expect to find the 

 cavities between the quartz-grains filled up with quartz, since the 

 associated veins clearly show that it has been largely introduced 

 into the rock since deposition. There is, however, often little or 

 no evidence of any such extensive introduction of mica ; so that I 

 doubt if we ought to look upon it as at all probable that mica 

 could have been so deposited chemically as to fill in the spaces 

 between them and complete crystals mechanically deposited, and 

 not leave some well-marked evidence of such a double origin. 

 However, on most carefully studying the character and arrange- 

 ment of the mica in schists, I saw that the crystals, which are 

 often of considerable size, are fitted and dovetailed together in the 

 most complicated but yet most accurate manner, just as if they 

 had crystallized in situ. The individual crystals are well defined, 

 because they are highly dichroic ; and by using a polarizer alone 

 they show very marked differences in tint, due to their different 

 orientation, and nothing to indicate a second growth. I can 

 scarcely believe that any one would contend that the mica of those 

 schists which possess cleavage-foliation had been deposited mechani- 

 cally. If such a view could be maintained, it would be in the case 

 of stratification-foliation ; but even then the manner in which the 

 crystals interfere with one another is just as it is in schists vdth 

 cleavage-foliation, or even in granite itself. The crystals of horn- 

 blende and other minerals also interfere in the same manner. 



Taking, then, all the facts into consideration, there appears to be 

 very complete proof that the principal constituent minerals of the 

 schists, from the districts examined, were not deposited mechani- 

 cally, but were formed or recrystallized in situ. In some cases 

 there is satisfactory evidence to show that previous to the crystal- 

 lization the rock was analogous to normal slates ; and even when 

 there is no such positive evidence, there is nothing against this 

 supposition, but much in its favour. As far as my observations 



