What is a porphyritic granite layer in sediment?

Question

I'm looking at what I'm told is porphyritic granite. The wall is sediment laid down perfectly flat but this granite cuts through it not quite flat, at a bit of an angle. The sediment is normal above and below it. What is it and what caused it???

Answer 1

Granite is an igneous intrusive rock, so it sounds as if you are looking at a d**e of granite.  Porphyritic means it has some larger crystals in a finer groundmass.  If it were entirely parallel the sedimentary layers it would be a sill.  The presence of a granite d**e suggests that there will be a larger granite intrusion somewhere not to far off.