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Quoting Wikipedia, Market cap is given by the formula
MC = N × P
where N is the number of shares outstanding, and P is the closing price per share.
In the specific case of a crypto currency, one could assume that N is the amount of coins in circulation and P its current value.
How can the image below happen, given that Zcash, being a fork of Bitcoin, cannot (unless I missed something very important) be destroyed, thus N can only grow.


2Did you notice in the graph that price is measured in BTC, while market cap is measured in dollars? Your units are inconsistent. Everything in the picture would be completely consistent if the price of BTC (in dollars) fell during the relevant time span. – Nate Eldredge – 2017-01-11T15:09:31.790
@NateEldredge: Thanks, I had missed that. I've updated my answer. – Murch – 2017-01-11T16:05:18.973