Portion of streets may be set aside as parks.

The proper authorities of the District are authorized to set apart from time to time, as parks, to be adorned with shade trees, walks, and inclosed with curbstones, not exceeding one-half the width of any and all avenues and streets in the said City of Washington, except Pennsylvania Avenue, leaving a roadway of not less than 35 feet in width in the center of said avenues and streets or 2 such roadways on each side of the park in the center of the same; but such inclosures shall not be used for private purposes.