As indicated by the caption, this image shows the velvet draped writing table in the Mackay mansion library as it appeared in 1902.
The caption accompanying this photograph reads as follows:
1902: The velvet draped writing table
Reading this, made perfect sense, as no illustrations showing a room with book shelves appears in the 1915 monograph. The photo above however was used in articles published to announce Harbor Hill's completion. They include one from the New York Herald on Sunday, August 10, 1902 and "The Founding Of An American Estate" in the August 16, 1902 issue of Town and Country magazine. The 'stone room,' is nowhere to be seen, but, perhaps the library could have been behind the hall. Where would the service wing's stores and offices have been located though, if that had been so? It's the laws of physics make this explanation impossible.
There are six other photographs of the Mackay mansion library that have been placed here in the Library,the first of which can be seen here, the second of which can be seen here, the third one can be seen here, the fourth one can be seen here, and the sixth one can be seen here.
See Harbor Hill, Clarence Hungerford Mackay's Mansion.
