Igneous Rock is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. Magma that erupts onto the surface is called lava. When magma cools slowly underground the crystals are large enough to see.
Readings for compression tests usually vary between 175 to 250 PSI (Pounds per Square Inch). A higher reading generally indicates better engine performance; however you should check your vehicle manual to find out what the ideal reading should be for your vehicle.
Connect the tester to a cylinder and turn the engine over about 6 to 8 compression strokes. You will be able to hear the cranking speed slow as the tested cylinder comes up on it's compression stroke. Note how fast the compression increases and jot down the highest reading.