be + of介词短语作表语，为正式语体。be of +名词，相当于be +形容词。但有时名词没有同源形容词时，就用be of 结构来表示主语的特征或性质。例如：

The book is of some use to you.= The book is useful to you.

The statements were of political character.

The buildings around are mostly of modern constructions. 周围这些建筑大多具有现代建筑风格。