Conversations with Professor Prestwich #3
CROSS CUTTING RELATIONSHIPS and IGNEOUS INTRUSIONS

Dear Geoneophytes,

One way to unravel the web of earth history is to record the timing of events by looking for cross cutting relationships. In this conversation I will address igneous events during which magma has forced its way into various relationships with sedimentary and metamorphic rocks. From Skye notice dykes that are vertical and intrude perpendicular to layers of sandstone. Please contrast these with the three sills from Trotternish that are more or less horizontal and intrude parallel to layers of sandstone.

A more complicated situation is illustrated by a sketch of two types of vein: granite and quartz that both cut metamorphic slates in Cornwall southern England. Finally, students, observe the relationships depicted in the sketch from Northwestern Scotland in which tongues of granite intrude metamorphic schist. Evidence of this sort was crucial in establishing the true nature of granite in the debates of long ago.

Cordially,

Joseph Prestwich