REVIEWS 639 



every gradation. Dynamic nietamorphism, contact phenomena, folds, 

 faults, thrusts, shearing, and foliation are all presented over and over 

 again. The unraveling of the history of such a region must be the 

 result of long and patient detailed work. Only after most elaborate 

 field and laboratory investigations can facts of broad bearing be 

 enunciated. 



It was in this region that the late Professor Williams did by far 

 the larger portion of his life's work, and he had a knowledge of it 

 which no other man has ever had. The results of many of his studies 

 have been already published, and so we have his papers on the gabbros 

 and his beautiful map of the Baltimore region. The larger generaliza- 

 tions which only come after many years of study were in many cases 

 not yet completely formulated, and in others, while formulated, were 

 unpublished. 



In the present paper we have Professor Williams' views as to the 

 origin of the granites and pegmatites. As an introduction to the 

 former he has summarized the criteria for the recognition of ancient 

 plutonic rocks in highly metamorphosed terraines ; a summary which 

 is most valuable, though marred by the incompleteness of the refer- 

 ences. Among other field evidences of the igneous origin of doubt- 

 ful rock masses are enumerated the presence of radiating apophyses, 

 foreign inclusions, and contact zones. That these may be obscured 

 is recognized, and in the recognition of altered eruptives the author 

 evidently relies largely upon chemical and petrographical evidence. 

 The test formulated by Rosenbusch and depending upon the definite 

 or indefinite character of the chemical composition of the rock has 

 been applied by Professor Williams to certain of the gneisses of the 

 region. Faint traces of structure originally igneous are found in 

 rocks which are quite completely changed. The development of 

 certain minerals is regarded as strongly suggestive of contact meta- 

 morphism. The determination of the relative ages of intrusions in 

 such a region must rest largely upon contact phenomena. Often an 

 eruption may prove to be anterior or posterior to some period of 

 strong metamorphism, and hence its relative place in the history of 

 the region may be known. It is by means of such evidence that the 

 origin of the granite masses has been tested. The detailed observa- 

 tions upon granites of the central portion of the state are recorded by 

 Dr. Keyes, and in general it may be stated that with few exceptions 

 the granites of the entire region may be proved to be of igneous 



