114 WORCESTER COUNTY HORTICULTURAL SOCIETY. [1893. 



altered mica which contained less iron. It came from the bank of 

 the Blackstone in Millbury, twelve feet below the surface. 



A third method is where the feldspar of our granites and gneisses, 

 under the action of water saturated with organic acids, loses its pot- 

 ash and soda, and either changes to other minerals, or breaks down 

 to a fine clay called kaoline. The quartz, if coarsely crystalline, 

 stands up in ridges under this action, or, if fine, breaks into sand. In 

 time the sand washes away and the clay rolls over a little and sticks. 



In the same way the sand and gravel is washed off from moraines 

 and covers the low ground or is transported by streams. (The hills 

 about Worcester are clayey, the valleys sandy) . 



The mechanical force of water, the expanding power of freezing 

 water, and the heat of the sun also are agencies at work in reducing rocks 

 to soils. Winds as well as water transport sand and scour the faces 

 of outcrops, and at the same time reduce to greater fineness the trans- 

 ported material. These used to be thought the most efficient agencies 

 in soil making. But in my judgment, except in the case of shaly 

 rocks, frost plays no very important part. 



Here in Worcester on Oak Hill is a glaciated surface of rock, never 

 very hard or compact, and the strata of which are presented almost 

 edgeways to the surface. The soil on that hill can never have been 

 deep, and now averages, for two or three acres on the top, not over 

 five or six inches in depth. But wherever thus covered, the glacial 

 grooves and scratches are scarcely weathered at all. They are nearly 

 as distinct as ever, and yet they cannot have been made much less 

 than 7000 years ago. 



At the same time another fragment, taken from a ledge in South- 

 bridge, was polished almost as smooth as glass. The upper surface is 

 intact. Two inches below it is crumbling to dust. The latter speci- 

 men was buried ; the former, from Oak Hill, was almost bare. A third 

 illustration is found in the character of the excavation for the founda- 

 tion of the main building of Clark University. Here the hydro-mica 

 schist, just the same rock as that which outcrops on the hill back of 

 the Oread Institute, was found, for several feet in depth, to be so 

 rotten that the shovel was easily thrust into it. Why should this 

 under four or five feet of soil be decomposed, while a really more 

 fragile rock on Oak Hill should bear its scars of seventy centuries. 

 There are two reasons : First, the glaciated rock is by the immense 

 weight and enormous grinding force solidified on the surface. Cracks 

 are filled. The surface is made water proof. Secondly, the growth 

 of vegetation there was scanty, the products of decay escaped into 



