How Rocks are Formed. 161 



against each other gradually reduce these to a state of sand or clay, and 

 in this way are produced the materials which make up the sands ones 

 and conglomerates. These, by the various changes which are taking 

 place in the earth's surface, become buried under other deposits 

 and are acted upon by the agencies of heat, pressure and other causes 

 till they become firm and enter into the solid constituents ot the earth's 

 crust. The softer muds and sills of the beach also undergo a change 

 and pass into shales. This material is deposited under quieter con- 

 ditions, in sheltered bays or creeks, where the finer earth particles held 

 in water, are gradually deposited. Shales pass into slates through the 

 formation of cleavage planes which have been induced by pressure in 

 the shaly mass, and by hardening through metamorphic agencies. 

 Wherever organic life has existed on the beach or shore, these remains 

 gradually become entombed and we now fir.d the impression of the long 

 extinct bird, fish, plant or insect, often so perfectly preserved that the 

 most delicate points of structure can be readily determined. These 

 organic remains are found to vary in character at different horizons, so 

 that what are found in one rock series often do not appear in others more 

 recent ; and upon this peculiarity of distribution, palaeontologists 

 and geologists have built up a scheme of rock formations, which com- 

 prises all the sediments from the Laurentian time or the original deposi- 

 tion of the earth's crust, down to the present day, each division of 

 which is distinguished by certain fossil forms peculiar in large part to 

 itself. In this way we can depict the whole life history, of the globe, 

 from the advent of the first forms, through plant, fish, bird, reptile, etc., 

 to the mammalia, and up to the highest type of all the genus Homo, or 

 man himself. 



While, however, sedimentary rocks are deposited as sands, clays or 

 calcareous matter in generally horizontal attitudes, such as we see in 

 the strata surrounding this city, very frequently these strata are tilted at 

 all angles, and in some cases completely overturned. This change in 

 position is accompanied often by a change in the character of the original 

 sediments, and is due to some agency, either of contraction or shrinking 

 of the crust or to dislocations which have produced crumplings, 

 upheavals, displacements, etc. In this way sandstones have been 



