The first edition of this book has been out of print for seven years. The ques tion as to whether a new edition should be produced was answered affirmatively on many counts. I think that the considerations which led me to write this book in 1949 are still valid (see Preface to the First Edition). Moreover, a description of those areas of interest which together comprise the field of Mineralogy seems to be more necessary than ever, because of the rapid advances which have been made. Due to the rapid extension of our knowledge, I did not dare again to treat the whole field by myself. Accordingly, Professor ZEMANN kindly agreed to revise the first part of the book dealing with Crystallography. He made many important corrections. In Part II the basic question arose as to whether the physical-chemical approach to rock forming processes, becoming more and more important, required inclusive treatment of the fundamentals of physical chemistry in the book. I see certain dangers in trying to produce a petrology text which is physical chemically self-sufficient. Thus, I retain the same opinion which prevailed when I wrote the previous edition; namely that the necessary basic knowledge should be acquired in lectures and laboratory classes in physics, chemistry, and physical chemistry, and with the help of standard literature dealing with these subjects. This back ground is, therefore, presumed and fundamentals are only referred to occasionally.