Import Data to CRM

Hi all,

I have data to import to MS CRM, In that files I have  Last Modified by,  Last Modified on fields. While mapping these 2 fields with CRM modified on and Modified by, I am unable oi find those 2 fields here. Is there any specific reason for this.? Created on, Created by fields also available (In case anyone thinking modified fields are read-only)

Please suggest me

Thanks in advance,

Shivaram