Short Papers on Geologic Subjects: Present State of Knowledge Regarding the Pre-Cambrian Crystallines of Illinois; Factors Affecting Measurement of Permeability of Unconsolidated Glacial Material; Fac

A hands-on look at Illinois’ ancient crust and what lies deep beneath the surface.

This collection presents field notes, maps, and petrographic observations from boreholes that penetrate pre‑Cambrian crystalline rocks, revealing how rocks differ across regions and how they change with depth.

From the northern part of the state to Pike County, the report contrasts red and gray granites with rare altered felsite found in dikes, and it documents granophyre and rhyolite porphyry as distinct rock types. It traces how hydrothermal fluids can alter feldspar and surrounding minerals, turning felsite into soft green clay and leaving clues in mineral patterns, texture, and color.
The text also covers drilling methods and sample handling, explaining how tool type, hole size, and mud influence the material brought to the surface and how carefully a driller must work to preserve representative samples.
Readers will find:

• Detailed descriptions of rock types, textures, and key minerals, including quartz, feldspar, biotite, titanite, and accessory minerals.
• Notes on how rocks differ between northern Illinois granites and southern rocks such as rhyolite porphyry and granophyre.
• Observations on how drilling techniques and hydrothermal alteration affect sample quality and interpretation.
• Context on the broader Illinois crust, faulting, and how post‑Cambrian processes may have modified earlier terrain.

Ideal for geology enthusiasts and students who want a concrete look at field methods, rock textures, and the deep history of Illinois’ bedrock.