definitions/feature_rules/delta_feature.json

{
  "format_version": "1.13.0",
  "minecraft:feature_rules": {
    "description": {
      "identifier": "minecraft:delta_feature",
      "places_feature": "minecraft:delta_feature"
    },
    "conditions": {
      "placement_pass": "surface_pass",
      "minecraft:biome_filter": [
        {
          "test": "has_biome_tag", "operator": "==", "value": "basalt_deltas"
        }
      ]
    },    
    "distribution": {
      "iterations": 50,
      "x": {
        "distribution": "uniform",
        "extent": [ 0, 16 ]
      },
      "y": {
        "distribution": "uniform",
        "extent": [ 0, 124 ]
      },
      "z": {
        "distribution": "uniform",
        "extent": [ 0, 16 ]
      }
    }
  }
}