55° THE JOURNAL OF GEOLOGY. 



are two very distinct stages. The one consists of explanation 

 of the immediate phenomena in hand. This gives the laws of 

 the phenomena, and may be called the Formal Theory. The 

 other explains the cause of these laws, and may be called the 

 Causal or Physical Theory. All science passes through these two 

 stages. For example : Until Kepler, the phenomena of Planetary 

 motion were a mere chaotic mass of observed facts without 

 uniting law. Kepler reduced this chaos to order by the dis- 

 covery of the three great laws which go by his name. This is 

 the formal theory of Planetary motion. But still there remained 

 the question, why do planets move according to these beautiful 

 laws ? Newton explained this by the law of gravitation. This is 

 the causal or physical theory. 



But this is so important a distinction that I must illustrate by 

 examples taken from geological science. All the phenomena of 

 slaty cleavage are completely explained by supposing the whole 

 rocky mass to have been mashed together horizontally and 

 extended vertically. This is the Formal theory and may be regarded 

 as certain. But still the question remains : How does mashing 

 produce easy splitting in certain directions? The solution of this 

 question is the Physical theory, and is perhaps a little more doubt- 

 ful, though I think satisfactorily answered by Tyndall. But still 

 there remains a deeper and more doubtful question, Whence 

 is derived the mashing force ? Is it general interior contraction, 

 as some think, or is it local expansion as others think. A perfect 

 theory must answer all these questions. Take another example : 

 All the phenomena of the drift are well explained by the former 

 existence of an ice sheet moving southward by laws of glacial 

 motion, scoring, polishing, and depositing in its course. This is 

 the Formal theory. But still the question remains, What was the 

 cause of the ice sheet ? Was it due to northern elevation, or to 

 Aphelian winter concurring with great eccentricity of the earth's 

 orbit ? And if due to northern elevation, what was the cause of 

 that elevation ? A perfect theory must answer all these ques- 

 tions. Take one more example : All the phenomena of earth- 

 quakes are completely explained by the emergence on the surface 



