Why does ProgressBar hide behind the button?

I created a RelativeLayout with a button and a circular ProgressBar. Here's how:

<RelativeLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="center_horizontal">

  <Button
    android:id="@+id/btnLogin"
    android:theme="@style/ButtonStart"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/login" />

  <ProgressBar
    style="?android:attr/progressBarStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_centerVertical="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_marginLeft="14dp"
    android:layout_marginStart="14dp"
    android:id="@+id/progressBar4"/>

</RelativeLayout>

Regardless of the order, first entering Button and then ProgressBar , vice versa, after the compiled project, ProgressBar is always behind the button. Why does ProgressBar hide behind the button? What is the best way to solve this problem?