Link to the Colorado Department of Personnel and Administration: Historical Records Index Search
http://accipiter.state.co.us/archive/publicrecordsearch.do
I found several records for Van Valkenburg:
Record Type: Probate
Name: Van Valkenburg, Galen B.
County: Boulder
Year: 1896 Month: Day:
Origin: Boulder County
Case No.: 1032
Record Type: Governor Appointments
Name: Van Valkenburg, R.J.
County:
Year: 1880 Month: May Day: 15
Origin: Governor
Governor: Pitkin
Kind of Appt: Notary Public
Book: 3; Page: 394
Access to these is through email to the department and includes a fee. I have not yet tried to get copies.