PACINI, METAMORPHISM OF PORTLAND CEMEST 185 



Other divisions have already been so thoroughly covered by previous in- 

 vestigators that very little remains to be said about them. Emphasis has 

 therefore been laid in this paper upon the little known fields. 



The problems which confront the user of concrete are of a high order 

 of complexity.. The generalizations of chemistry are not yet sufficiently 

 developed to apply rigidly to systems of so many variables, and experi- 

 mental work on a laboratory scale often fails almost entirely to reproduce 

 the conditions of practice. The best guide to the truth, then, is the prag- 

 matic sanction of experience the investigator in this field can but point 

 out probable directions for future experimentation. The theories which 

 underlie past success are a safe guide, nevertheless, to future construc- 

 tion, and the systematization thereof is a legitimate field of usefulness. 



While, strictly speaking, any aggregation of chemical compounds 

 might be considered a rock, whether natural or artificial, a majority of 

 the cases conceivable under such a classification would not present im- 

 portant petrological problems in the study of their metamorphism. Such 

 a problem as the action of water upon a mixture of sodium chloride and 

 calcium sulphate can be partly solved in vitro, even though the action 

 of sea water upon gypsum deposits is an interesting petrological investi- 

 gation. 



The important components of Portland cement are everywhere about 

 us in nature, and the reactions by which it is made artificially have been 

 taking place for many geological ages without the intervention of man. 

 Silica, alumina and lime are among the most important constituents of 

 the earth's crust; they are subjected in places to the same conditions 

 that exist in the kiln, and are afterwards acted upon by water, under 

 some of the same conditions under which man builds massive structures. 



The complex question of the history of rock magmas is not one to be 

 solved by any one group of scientists, but by patient and concerted efforts 

 of the chemist, the physicist and, above all, the petrologist. So the prob- 

 lem of the constitution of Portland cement may be as yet somewhat inde- 

 terminate ; but an examination of the more general effects of metamor- 

 phism may reveal some identity with conditions in natural rocks already 

 studied and may direct us to the correct methods for investigation of the 

 constitution of cement (67). 



Other important problems in the field of cement and concrete are re- 

 ferred to in the following pages, and belong in great measure to the field 

 of petrology. Not the least important of these is the suitability of vari- 

 ous types of rocks for use as aggregates in concrete, and this work is 

 claiming more widespread attention daily (19, 44, 111). 



