The Ottawa Naturalist. \Ap 



Vol. IX. OTTAWA, NOVEMBER, 1895. No. 8. 



HOW ROCKS ARE FORMED. 



By R. W. Ells, LL.D, F.R.S.C. of the Geological Survey of Canada. 



Before taking up the general subject of rock formation, which in 

 the limited time at my disposal, can only be touched upon in the 

 briefest possible manner, we may for a moment glance, first of all, at some 

 of the theories which have been put forth to account for the formation 

 of the earth itself, in order that we may obtain a good starting point or 

 acquire some idea of the conditions under which the foundations of the 

 earth's crust were laid down, upon which the many thousands of feet of 

 rock material which are known by the names of sandstone, slates, shales 

 and limestones have been deposited. 



Many theories have been put forth to explain the formation of the 

 earth and to account for the many changes which transpired thereon 

 before it became fitted for the advent of animal and plant life. Of 

 these some are of interest from their legendary character, while others, 

 regarded from the standpoint of modern science, present many features 

 not reconcilable with the knowledge of the present day, and are of value, 

 chiefly as illustrating the crude ideas that prevailed on this subject, 

 prior to the advent of the present century. But few of the propounders 

 of these theories made any attempt to approach so complicated a prob- 

 lem from a purely scientific standpoint. It must be borne in mind that 

 the scientific study of the earth's crust is a matter of comparatively 

 recent date, and our present knowledge is the result of very careful 

 study, both in the field upon the rock masses themselves and in the 

 laboratory, in which the science of chemistry and the microscope have 

 played very important parts. 



According to the theory now most generally accepted regarding 

 the formation of the crust of the earth, viz., that of Laplace, there un- 



