1.  Marriage Record:

2.  1860 Census Index/Images (online at Genealogy.com):  can't find, and I did a page-by-page search of Susquehanna Twp. and Delaware Twp.

3.  1870 Census Images (online at Ancestry.com; Image #8 of 23):  Mahontengo P.O., Susquehanna Twp., Juniata Co., PA, p. 465B, PN 8, 53/53, enumerated 13 Aug 1870, official enumeration date 1 Jun 1870 (extracted by Diana Gale Matthiesen):¤•

Dressler Joel	41	M	W	Farmer	2000	600	Pa
________ Kate	30	F	W				Pa
________ Alfred	16	M	W	L $			Pa
________ Sarah	14	F	W				Pa
________ Daniel	12	M	W				Pa
________ Jerimiah	10	M	W				Pa
________ Agnus	1	F	W				Pa

Joel DRESSLER is Kate's second husband, and the three eldest children are Kate's by her first marriage, to Mr. ZELLERS — by the way, we know that Alfred used the surname ZELLERS, not DRESSLER.  If Jeremiah is 10, as it says here, then he has to be a son of Mr. ZELLERS.  If he is 3 years old, as indicated by the 1880 census, then he is Joel's biological son, not his adopted son.  Catharine should be age 36, not 30.  Living three doors from Michael & Eva Maria STROUP. 

4.  1880 United States Census and National Index (LDS Family History Resource File: CD-ROM Library):¤•

Census Place: Susquehanna, Juniata, Pennsylvania
Source: FHL Film 1255137; NARA Film T9-1137; Pg 360A
	Relation	Sex	Marr	Race	Age	Birthplace	Occupation	Fa	Mo
Joel DRESSLER	Self	M	M	W	52	PA	Farmer	PA	PA
Catherine DRESSLER	Wife	F	M	W	46	PA	Keeping House	PA	PA
Jeremiah E. DRESSLER	Son	M	S	W	13	PA	Works On Farm	PA	PA
Jasana A. DRESSLER	Dau	F	S	W	11	PA	At Home	PA	PA
Joel T. DRESSLER	Son	M	S	W	9	PA		PA	PA
Louisa C. DRESSLER	Dau	F	S	W	7	PA		PA	PA
Amelia U. DRESSLER	Dau	F	S	W	5	PA		PA	PA

If Jeremiah really is 13, then he was 3, not 10, in 1870, making him Joel's son, not Mr. ZELLERS's.  The fact that Joel calls him "son" is not proof he's Joel's biological son, because I know of many cases where adopted children are not so indicated in the census.