are mostly of疑问

The building around are mostly of modern constructions.

are of有如此的结构？

be + of介词短语作表语，为正式语体。be of +名词，等于be +形容词。但有时名词没同源形容词时，就用be of 结构来表示主语的特点或性质。比如：

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.周围这类建筑大多具备现代风格。