The study of rocks Author:Frank Rutley Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: five to thirty miles is about the smallest estimate which has been given ; while some, as Hopkins, have inferred that solid matter extends to much greater depths... more » and may even exist at the centre, the loss of heat and consequent solidification having taken place in an irregular manner, and having thus converted the deeper portions of the globe into a somewhat honey-combed mass, the cavities still retaining matter in a molten condition, and constituting the reservoirs from which the eruptive rocks are derived. So many theories upon this subject have from time to time been started, and they embody such diversity of opinion, that a description of them would be out of place in so small a book; and as they cannot be regarded as more than speculations, often based upon a little tangible truth and more or less tangible and intangible error, students, although doing well to make themselves acquainted to some extent with these theories, would do better in giving their attention to matters which are more readily demonstrable. That the forces just spoken of as existing in the interior of the earth exercise considerable influence upon its crust we have ample evidence, it is, indeed, solely from such evidence that we infer the existence of the forces. The evidence which we have of the internal heat of the earth may be summed up under the following heads :— (1) In descending the shafts of mines a gradual rise of the thermometer takes place after the descent of the first sixty feet. Down to this point it remains stationary; below this point there is a rise of one degree Fahrenheit for every sixty feet descended. How far this regular increase of temperature continues to take place has not yet been determined. (2) Flows of molten lava and of hot mud, the ejection of lapilli and ashes fr...« less