W. M. Hutchings — Clays, Slates, and Contact- Metamorp/mm. 69 



any but very moderate thermal or other effects, due to its intrusion, to 

 have acted ; and this conclusion is borne out by the very slight effects, 

 if any, which can be seen to have taken place in other particulars. 

 The sedimentary rock being under those conditions of temperature 

 and pressure due to its depth of cover, etc., the intrusion of the 

 igneous rock may have slightly intensified the temperature at these 

 points, — possibly also the pressure, for a time. But this has sufficed 

 to bring about the alteration of the rutiles. 



We have already considered the question as to liow the change is 

 brought about, and simple solution of the rutile appears to be the 

 necessary process. The intrusion of igneous rock appears to have 

 given rise to this, and as other effects are absent or so slight, we 

 need not assume anything more than water, or perhaps a weak 

 alkaline solution, to have come into play. The manifold forms in 

 which the dissolved titanic acid may recrystallize have been pointed 

 out, and we may now add brookite as a further extension of this 

 list, E. Beck having observed its occurrence in the rocks of the Elbe 

 Valley contacts, in a manner exactly similar to that of the Anatase 

 at Shap.^ 



(2) Biotite. — Perhaps the next mineral change, in degree of 

 sensibility, is the appearance of biotite in the slates undergoing 

 contact-metamorphism, which is also a matter of universal obser- 

 vation. It takes place at considerable distances from the contact, 

 and may be seen already far advanced before the formation of 

 regenerated quartz-mosaic and other minerals is demonstrable. 



A large number of slates, etc., contain a good deal of chlorite, which 

 was deposited in them at a period subsequent to their formation as 

 such, by simple infiltration processes. This chlorite is altered into 

 biotite. 



This change, like the one last considered, also cannot be explained 

 by direct action of raised temperature or pressure. No amount of 

 simple "molecular rearrangement" of chlorite under heat or pressure 

 will make biotite of it. A portion of its water could be removed, 

 as is required ; but it has also to take up potash, and here again 

 we must look to solutions, which in this case must contain some 

 alkaline salt. 



There are, again, many slates, shales, etc., which are wholly or 

 almost free from chlorite, but which contain a sufficient amount of 

 magnesia to correspond to a good deal of biotite, this magnesia 

 appearing to have entered into combination in the new micaceous 

 minerals existing in the slate, etc. Under contact-action biotite may 

 be, and is, developed in such slates, and this could take place by 

 easily conceivable molecular rearrangements. 



Whether it be possible for flakes of chlorite, lying in among flakes 



1 Even this list of changes does not exhaust the number of transformations the 

 titanic acid may undergo. At later stages of the metamorphism it appears that part 

 of it can be taken up into the newly-formed contact-biotite, thus ending up where it 

 began, — combined in dark mica. Such contact-biotite has been shown, by Lang and 

 Jannasch, to contain as much as 3.40 per cent, of titanic acid, and Beck records 

 {op. cit.) cases of decayed contact-biotite in which rutile has once more been 

 separated out. 



