Dissolving shapefile but retain attribute fields using ArcGIS Desktop?

You just need to split it into 2 steps:

1. Dissolve on the NET_ID
2. Spatial Join the dissolved layer to the original layer. Use the match type of CONTAINS and set the Merge Rule of the Geology field to Join and set the delimiter to a comma. Right click on the field in the field mapping list and select properties to get to the merge rule and delimiter settings