It is clearly very fine-grained, which makes it difficult to ID the rock type: could be mudstone, could be rhyolite; other possibilities. However, much of the structure is due to narrow sedimentary layers and veins of a slightly harder mineral, quite possibly quartz.
The dark rhombic shape show in SAM_0771.JPG could be a pyrite crystal.
The overall colour (including the mottling) is fairly unusual. I would be thinking of arid sedimentary environments, or hydrothermal metamorphisis as possible explanations.