Currency type in CurrencyZone sample

Suppose a Scala class has a type which is defined in a subclass. How it should be referred from another class?

Below is the CurrencyZone sample from the Scala book.
Which type should I use for the argument of a method log(currency:???) that just prints an amount of the given currency?

Something like this:

object CurrencySample {
  def main(args: Array[String]): Unit = {

  def log(currency: /* Which currency type should be used there?*/): Unit = { }
abstract class CurrencyZone {
  type Currency <: AbstractCurrency

  val CurrencyUnit: Currency

  def make(amount: Long): Currency

  abstract class AbstractCurrency {
    def designation: String

    def amount: Long

    def +(that: Currency): Currency = make(this.amount + that.amount)

    def *(number: Double): Currency = make((number * this.amount).toLong)

object Europe extends CurrencyZone {
  val Cent = make(1)
  val Euro = make(100)
  val CurrencyUnit = Euro

  type Currency = Euro

  override def make(cents: Long) = new Euro {
    override def amount = cents

  abstract class Euro extends AbstractCurrency {
    override def designation = "EUR"

You can use something like:

object CurrencySample {
  def main(args: Array[String]): Unit = {
    // log(US.Dollar)

  def log(zone: CurrencyZone)(currency: zone.Currency): Unit = { }

Thanks a lot. This really what I am looking for.