Jim neglected to point out the the 2nd sentence is not grammatically correct.

vishalwin wrote: 1) Ron's letters to Mary, which were written over a period of beginning a few years before Susan's marriage and ending to Emily's brother and ending shortly before Emily's death in 1886, outnumber her letters to anyone else.

In this 1st example, the which is allowed to "hop" over a prepositional phrase to modify the noun before it - in this case, "letters." We can think of the "which" clause as modifying the NOUN PHRASE before it. (By the way, this sentence is a strange half-adaptation of the famous OG Emily Dickinson problem.)

For more on WHICH "hopping" over essential modifiers, see:
https://www.beatthegmat.com/written-in-i ... tml#797357
https://www.beatthegmat.com/og13-q70-the ... tml#782099
https://www.beatthegmat.com/marconia-s-c ... tml#774776
https://www.beatthegmat.com/question-in- ... tml#767098
https://www.beatthegmat.com/og13-q47-in- ... tml#791448
https://www.beatthegmat.com/gmat-prep-in ... tml#787888

2) Because of plunging prices for computer chips, which is due to an oversupply, the manufacturer has announced that it will cut production.

This sentence is grammatically incorrect. The "which" could theoretically refer to either "plunging prices" or "computer chips," depending on the context. However, "which is" tells us that the antecedent must be singular. Since there is no singular noun available for "which" to replace, the sentence doesn't work.