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



igneous masses, we find that here, too, a development of the mica 

 and a separation of chloritic matter have taken place, with a more or 

 less considerable advance in the crystalline structure of the rock, 

 up to very high stages indeed, even when none of the special 

 " contact- minerals " are yet formed. 



The consideration of these different cases brings us face to face 

 with the most interesting, and as yet quite unsolved problem, as to 

 how far such rock-development can be shown to go without calling 

 in the aid of " contact-metamorphism," — the only agency of which 

 we really know that it is capable of carrying the crystalline develop- 

 ment of these sedimentary rocks to its highest degree. There are 

 three agencies by which we may assume that such crystalline slates 

 or phyllites may be produced, and in many cases we may have very 

 little or no direct proof or evidence as to which of them has been the 

 cause. These three agencies are dynamic action, contact-action, and 

 finally another action which is neither dynamic nor contact, but is 

 simply due to long-continued submission to depth-temperature and 

 pressure in deeply-buried sediments. 



As regards the first of these agencies, the dynamic, it is, perhaps, 

 really not at all possible to consider it alone. In all rocks which 

 have been subjected to it, we must take into the question the 

 amount of influence which the third agency has possibly exerted, 

 both previously to, and concurrently with, the actual dynamic 

 action. 



Then, again, contact-action will often be due, and in its most 

 striking developments will probably always be due, to the effects 

 of intrusions on rocks which have already been a long time under 

 the influence of more or less considerable depth-conditions, and 

 which have undergone whatever changes these conditions may be 

 ca|3able of causing in them. 



No problem of petrology is of deeper and wider interest than that 

 involved in the question here concerned ; none demands more 

 careful study or more freedom from hasty bias ; and none has been 

 more prejudiced by immense and authoritative generalizations on 

 insufficient data. 



One point, I think, is worthy to be insisted upon. It is of the 

 greatest importance that investigation of the higher phases of 

 development of these rocks should be based, first of all, upon a 

 minute study of the lower forms of them in clays and shales, so as 

 to understand as fully as possible what it is from which the 

 evolution starts ; what is the condition, chemically and miueralogi- 

 cally, into which the sediments have passed at the earliest stage 

 of their history at which we can begin to regard them as having 

 become rock-bodies at all. 



Taking, firstly, dynamic action (but bearing in mind what has 

 just been said as to its probable association in all cases with other 

 influences), the tendency has been, among many geologists, to accept 

 it as capable of causing almost anything in the way of crystallization 

 and regeneration of sedimentary materials. It has even been made 

 to include, as it were, other agencies within it, contact-metamorphism 



