The family can be found in Coral Twp., McHenry Co, IL, in 1900 [name incorrectly entered as "ZARK" in the census list]. John and his 9 siblings reside with their parents. 10 out of 16 (!) children are still living at that time:
- Mary (b. Jan.1879)
- Eda (= Ida) (b. Oct.1883)
- August (b. Dec.1880)
- John (b. Apr.1884)
- Edwin (= Edward) (b. Jan.1887)
- Elizabeth (Louise ?) (b. Oct.1888)
- Minnie (= Wilhelmine) (b. Mar.1890)
- Carl (b. Dec.1892)
- Lydia (b. Aug.1894)
- Martin (b. May 1897)

In the 1910 U.S. Federal Census, he and his brother August O. ZARTH are listed together with the family of his sister Ida M. ARCHER in Tacoma, Pierce Co, WA (Ward 6). The rest of the family has settled in Core (Cove ?) Precinct, King Co, WA [family name incorrectly transcribed as "TARTH" and "LARTH"].

In November 1918, he lived in Ketchikan City, Alaska, and worked as a laborer for the "Vermont Marble Co," Tokeen, AK. (The company was one of the largest marble producers in the world with their headquarters in Proctor, VT. It contributed marble, for example, to Arlington National Cemetery.)

Later he and his parents are in Cove, King Co, WA (1920), finally moving to Vashon, King Co, WA (1930), where he can still be found together with his mother and his two brothers Edward and Martin in 1940.

He was the last of the ZARTH brothers to pass away.

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Sources:
- "Cook County, Illinois, Birth Certificates Index, 1871-1922"
- U.S. Federal Censuses (1900 [Coral, McHenry Co, IL], 1910, 1920, 1930 and 1940 [King Co & Pierce Co, WA])
- "U.S., World War I Draft Registration Cards, 1917-1918"
- "U.S., World War II Draft Registration Cards, 1942"
- "Washington Death Index, 1940-1996"
- www.familysearch.org (individual family trees)