请看这两个例子：
All marble is composed of crystals of the minerals calcite or dolomite,and which are perfectly white,when pure.

Lyn, the model, lay sprawled across a chair, her long leg clad in ragged denim, one sneakered foot tapping in time to the music which poured from the earphones planted firmly on her head--- and which were to remain there all the way to Egypt.
（and which were to remain ...的先行词是earphones,不是which poured...的先行词music)

这两个句子中都使用了非限定性定语从句，而且which前都带着一个and，但之前并没有与之并列的定语从句。

我觉得是否是因为之前句子太长，所以加and来缓和一下。

不知我的理解是否正确，或者有其他的解释。

请专家指点！！谢谢！

楼主的分析有一定道理。我个认为，不加and,说话人讲话是一气呵成，加上这个and，表示作者对自己的话作临时补充。这两种说法没有本质区别，前者表示说话人说话斟密，或早有准备，后者表示作者即兴说话。