According to Judd the Island of Skye in Inverness-shire is the remains of a volcano that was active in Tertiary times, probably many millions of years ago. This volcano was very large, probably about thirty miles across at its base, with a height of perhaps as great as 12,000 or 15,000 feet. Now there are only left some granite and other similar rocks that form the Red Mountains and Coolim Hills of Skye that reach about 3,000 feet above the sea level. There are many other parts of the world containing volcanoes that were active during the geological past. The above, however, is as far as we can describe such volcanoes in this book. CHAPTER XVIII LAPLACE'S NEBULAR HYPOTHESIS LaPlace's nebular hypothesis is the name given to an ingenious hypothesis proposed by LaPlace, a celebrated French astronomer, in an endeavor to explain how the solar system has been evolved. You will notice that this is called a hypothesis and not a theory. The word hypothesis is properly applied to a more or less intelligent guess or assumption, that has been made for the purpose of trying to find out in the cause of any natural phenomenon. A theory is an expression of a physical truth based on natural laws and principles that have been independently established. A theory, therefore, is much more complete than a hypothesis. A hypothesis, as Silliman remarks, bears the same relation to a theory or law, that a scaffolding does to a completed building, since it forms a convenient means for erecting the building. LaPlace's work is properly called a hypothesis, because it is not to be considered as any more than a means for enabling one intelligently to inquire into the probable manner in which the solar system has reached its present condition, by gradual steps or stages during the almost inconceivable length of time since its creation. Before describing LaPlace's hypothesis it will be necessary to give you some ideas concerning what is known by astronomers as the solar system. The solar system consists of the sun, and the eight large bodies called planets that revolve around the sun. It also includes a number of moons or satellites revolving around the planets, a number of small bodies, called planetoids or asteroids, together with numerous comets and meteorites. Besides these there is probably a system of meteoric bodies that are believed to revolve around the sun, and to produce, by the reflection of the light from their surfaces, what is known as the _zodiacal light_.