[求助]　请教og１９中的修饰成分？

Chinese, the most ancient of living writing systems, consists of tens　of thousands of ideographic characters, each character a　miniature calligraphic composition inside its own
        square frame.

请教：以上划线部分作为idepgraphic characters的修饰成分,　是Ｎ＋Ｎ＋prep的形式．有谁可以帮我解释一下这个形式做修饰成分的功能吗？

谢谢．