Bates (1) has Parks, but the descriptive roll (2) has Park.
He was 33 when he enlisted (3 [33], 7 [33]). He was born in Lathvan, Ireland (3, 7).
When he enlisted, he was a farmer (3, 7). He was living in Philadelphia when he was enrolled (7).
He was 5 feet 11 inches tall, had a light complexion, blue eyes, and light hair (3, 7 [5'10-1/2", light complexion, blue eyes, sandy hair]).
He was enrolled and mustered into service as a private in company B on 2 September 1861, at Philadelphia, Pennsylvania (1, 7).
He was transferred to company E (1, 7).
He was captured at Snicker's Gap, Virginia, probably early in November 1862 (5). He was reported missing in action (5). He had returned to the regiment on 22 February 1863 (5).
He fought in the battle of Gettysburg (4).
On 28 July 1863, he was reported as having been dropped because he was missing in the campaign (probably after 11 June 1863) (6).
He re-enlisted as a veteran volunteer on 26 March 1864, at Warrenton Junction, Virginia (3, 7 [30 Mar 64]). He was enlisted by Lieutenant Brass, and mustered into service by Captain Barnard (3).
He was wounded on 18 June 1864 at Petersburg, Virginia, and died on 23 July 1864 [burial record has 13 July according to (1)] from amputation of his leg as a result of those wounds (1, 2, 3, 7). He was buried in the National Cemetery, Arlington Virginia (1).
On 26 January 1865, company E published resolutions mourning his death (and fifteen others), and sympathizing with their families (8).
On 3 December 1864, his widow, Matilda Parks, successfully applied for a pension (9, 10).
1 Bates, Samuel Penniman. History of Pennsylvania volunteers, 1861-5. Harrisburg: B. Singerly, state printer, 1869-71. 5 volumes. 'Ninety-first regiment', volume 3, pages 186-233. (In the roster)
2 list of deaths, company E
3 descriptive roll, company E, entry 66 (John Park)
4 Pennsylvania Memorial, 91st Pennsylvania plaque, Gettysburg, Pennsylvania (John Parks)
5 consolidated morning report, 91st PA, 22 February 1683
6 consolidated morning report, 91st PA, 28 July 1863 (Private Parks)
7 Civil War Veterans' Card File, available at the Pennsylvania State Archives, searched 6 May 2004 (John Parks)
8 'Headquarters company E', Philadelphia Inquirer 26 January 1865 page 5 (John Parks)
9 pension index, by regiment, 91st PA infantry, company E (John Parks)
10 pension index, by name (John Parks)
[I did not find an entry for him in Ancestry's index to the 1890 veterans' census (searched 5 June 2009)]