def find_features(extent, df):
    """
    given an extent and a dataframe, return a new dataframe
    containing only features that fall within the extent

    Parameters
    ----------

    extent:  list -- geographic extent in lon (deg E)/lat (deg N)
    df:  the geopandas dataframe to slice
    """
    xleft, xright, ybot, ytop = extent
    hit_rows = df.cx[xleft:xright, ybot:ytop]
    return hit_rows