Sandstone dykes up to 15cm wide and more than 3 m long transect the bedding and are parallel to the cleavage in the Siamo Slate of the Marquette synclinorium. These clastic dykes are common in slates of very low metamorphic grade east of Negaunee, and are composed of fine-grained sandstones commonly containing a higher proportion of iron-rich carbonate than the host rocks. The cleavage in both the slates and the interbedded sandstones is produced by parallel orientation of thin pelitic foliae. Individual cleavage foliae can be traced from one pelitic layer, through a sandy lamination and into the next pelitic layer. Microstructures in disrupted sandy laminations indicate that the sandstone dykes formed penecontemporaneously with the cleavage. Both the cleavage foliae and sandstone dykes are interpreted as having formed by intrusion accompanying tectonic dewatering of the Siamo Slate during late diagenesis or very low-grade metamorphism.