Sounds like perlite or rock wool used back in the day. To be safe please do not touch it or move it and consult a local licensed insulation contractor.
What you have described without seeing a picture or reviewing the substance up close could be something else more hazardous to health or flammable.
Please check with an insulation contractor or your local code enforcement official. This would be my best advice without seeing what you have described.
Hope this helps.